Electroseismic surveying in exploration and production environments

ABSTRACT

Systems, methods, and computer programs for monitoring a drilling operation in a subterranean formation include receiving, from a first sensor array, one or more seismic signals caused, at least in part, by the drilling operation in the subterranean formation; receiving, from the first sensor array, one or more electromagnetic signals generated by an electro seismic or seismoelectric conversion of the one or more seismic signals caused, at least in part, by the drilling operation in the subterranean formation; and determining a property of one or more of the drillstring and the subterranean formation based, at least in part, on the seismic signals and the corresponding electromagnetic signals received from the first sensor array. The first sensor array is arranged to monitor the drilling operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claim priority to U.S. Provisional Application No.61/890,682 entitled “Electroseismic Surveying in Exploration andProduction Environments” by Arthur Thompson, Alan Katz, Robert England,Todd W. Benson, and Mark Griffin, which was filed on Oct. 14, 2014, thecontents of which are hereby incorporated by reference. This applicationclaim priority to U.S. Provisional Application No. 61/891,096 entitled“Electroseismic Surveying in Production Environments” by ArthurThompson, Alan Katz, Robert England, Todd W. Benson, and Mark Griffin,which was filed on Oct. 15, 2014, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

Conventional techniques for the control of down-hole operations may relyon various models, sensors, heuristics, and operator judgment todetermine, for example, the location of a drill bit in a subsurfaceformation or the propagation of fractures in the subsurface formation.These conventional surveying technologies, however, suffer from certainlimitations that may prevent a full understanding of the location andextent of down-hole operations. For example, particular surveyingtechniques may require the use of expensive and/or time consumingsurveying equipment and methods that may limit the economic viability ofsurveying a particular prospective region. In addition, particulartechnologies may be able to provide information regarding one or moregeophysical properties of a subsurface region, but may not be able toprovide information on other geophysical properties. Such limitationsmay lead to the identification of prospective regions for drilling orexploration based on an incomplete and/or incorrect understanding of theprospective region, which may cause unnecessary time and/or expenses tobe incurred exploring or drilling regions that do not have the desiredgeophysical properties. For example, based on incomplete or incorrectgeophysical surveying, a drilling operation may drill a dry hole ordrill into an unintended portion of the formation.

SUMMARY

In accordance with the teachings of the present disclosure,disadvantages and problems associated with conventional techniques ofdrilling a wellbore, propagating fractures, and producing a reservoirmay be reduced and/or eliminated. For example, a surveying system may beprovided using passive electroseismic or seismoelectric surveyingtechniques for well drilling, formation enhancement, and reservoirproduction. The surveying system may utilize survey data from passiveelectroseismic or seismoelectric surveying to monitor or control welloperations.

In accordance with one embodiment of the present disclosure, a methodfor monitoring a drilling operation in a subterranean formation includesreceiving, from a first sensor array, one or more seismic signalscaused, at least in part, by the drilling operation in the subterraneanformation. The method further includes receiving, from the first sensorarray, one or more electromagnetic signals generated by anelectroseismic or seismoelectric conversion of the one or more seismicsignals caused, at least in part, by the drilling operation in thesubterranean formation. The method further includes determining aproperty of one or more of the drillstring and the subterraneanformation based, at least in part, on the seismic signals and thecorresponding electromagnetic signals received from the first sensorarray. The first sensor array is arranged to monitor the drillingoperation.

receiving, from a first sensor array, one or more seismic signalscaused, at least in part, by the drilling operation in the subterraneanformation. The method further includes receiving, from the first sensorarray, one or more electromagnetic signals generated by anelectroseismic or seismoelectric conversion of passive source seismicsignals caused by the drilling operation in the subterranean formation.The method further includes determining a drill bit location based, atleast in part, on the signals received from the first sensor array. Thefirst sensor array of seismic sensors are arranged to monitor thedrilling operation. The first sensor array of electromagnetic sensorsare also arranged to monitor the drilling operation.

Technical advantages of certain embodiments of the present inventioninclude the ability to perform passive electroseismic or seismoelectricsurveying. Such surveying may detect an electromagnetic signal generatedin response to electroseismic or seismoelectric conversions caused bywell processes, such as drilling, production enhancement operations,e.g., fracturing, or reservoir production. Similarly, such surveying maydetect a seismic signal generated in response to electroseismic orseismoelectric conversions caused by well processes. The electroseismicor seismoelectric conversion may take place in a subsurface earthformation. Using such techniques, geophysical surveying may be performedwithout the requirement for expensive active sources of electromagneticor seismic energy, which may improve site safety and reduceenvironmental impacts. The reduction in the amount of equipment andpower, along with the corresponding reduced footprint at the measurementsite, may be an advantage over other surveying systems and methods. Froman environmental and health perspective, the reduction intransportation, site preparation, and high energy sources may improvethe overall health and safety of the workers operating the equipment. Inaddition, the electromagnetic field generated by well operationscomprises a broad spectrum of frequencies, from sub-hertz frequencies totens of thousands of hertz frequencies. This broad spectrum allows for abroad range of penetration depths from tens of meters to tens ofkilometers. This broad spectrum also permits high spatial and depthresolution. Accordingly, the electromagnetic and/or seismic signalsdetected may be processed to identify various properties of thesubsurface earth formation and the well operation.

Other technical advantages of the present disclosure will be readilyapparent to one of ordinary skill in the art from the following figures,description, and claims. Moreover, other specific advantages ofparticular surveying techniques and combinations are discussed below.Moreover, while specific advantages are explained in the presentdisclosure, various embodiments may include some, all, or none of thoseadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a perspective diagram illustrating an example system forpassive electroseismic and seismoelectric surveying;

FIG. 2 is a perspective diagram illustrating an example system forpassive electroseismic and seismoelectric surveying;

FIGS. 3A and 3B are flow charts of an example method of monitoringdrilling operations using techniques of the present disclosure;

FIG. 4 is a block diagram of a computing system according to the presentdisclosure;

FIG. 5 is a cross-sectional view of an example well and reservoir;

FIG. 6 is a cross-sectional view of an example well and reservoir wherethe plane-of-view is perpendicular to that of FIG. 5;

FIGS. 7 and 8 are cross-sectional views in a plane containing a well ina reservoir and the resulting electric field;

FIGS. 9 and 10 are graphs charting the vertical electric field for ahorizontal dipole at depth versus horizontal distance from a verticalplane passing though the lateral;

FIGS. 11 and 12 are graphs charting the amplitude of a horizontalelectric field versus horizontal distance from the center line;

FIGS. 13A and 13B are flow charts of an example method of monitoringfracturing operations using techniques of the present disclosure;

FIGS. 14A and 14B are flow charts of an example method of monitoringproduction operations using techniques of the present disclosure;

FIGS. 15A, 15B, and 15C are schematic diagrams of sensors according tothe present disclosure;

FIG. 16 is a flow chart of an example method of performingelectroseismic and seismoelectric surveying for a drilling operation;

FIG. 17 is a flow chart of an example method of performingelectroseismic and seismoelectric surveying for a fracturing operation;and

FIGS. 18, 19A, 19B, and 20 are flow charts of an example methods ofmonitoring production operations using techniques of the presentdisclosure.

DESCRIPTION OF EXAMPLE EMBODIMENTS

The example embodiments herein may utilize electroseismic and/orseismoelectric sensors to detect the electric fields and/or seismicwaves generated passively by well operation. Certain embodimentsdiscussed herein may use, at least in part, passive surveying techniquesthat utilize passive sources, such as naturally occurringelectromagnetic fields and/or seismic waves, and the interactions ofelectromagnetic or seismic signals generated by those sources withsubsurface formations through electroseismic and/or seismoelectricconversions to identify features and/or properties of subsurface earthformations. Such surveying may be useful for a variety of purposes,including the identification of subsurface water and minerals. Whilepassive surveying may be suitable for use as a standalone method ofgeophysical surveying, passive surveying may, in some embodiments, beperformed in conjunction with other geophysical surveying methods toidentify properties of subsurface earth formations. The teachings of thepresent disclosure are intended to encompass embodiments that employpassive surveying as a standalone surveying technique as well asembodiments that use passive surveying in conjunction with one or moreother methods of geophysical surveying.

A passive source may be utilized to provide the energy for generatingelectroseismic and/or seismoelectric conversions in a subsurfaceformation or structural feature. For example, the earth'selectromagnetic field and/or environmental seismic energy may induceelectroseismic or seismoelectric conversions in a subsurface earthformation that holds hydrocarbons or other minerals. As used herein, a“passive source” may include any source that is not being activelyinitiated by a surveying operation to actively generate a source ofseismic and/or electromagnetic energy. Although a passive sourcegenerally includes a natural source of electromagnetic energy and/orseismic energy such as the earth's natural electromagnetic field, otherman-made sources of electromagnetic and/or seismic radiation such aselectrical power lines or mechanical equipment may also be included aspassive sources in particular embodiments. While certain man-madesources may induce an electromagnetic field or seismic wave, they aredistinguishable from an “active source” such as a seismic generator,explosives, electric field generators, and the like in that such sourcesare generally initiated by and/or are associated with a surveyingoperation to facilitate surveying a subterranean formation. As usedherein, “passive surveying,” “passive electroseismic surveying,” and“passive seismoelectric surveying” may refer to surveying that utilizesa passive source as opposed to an active source. Passive surveying maydetect the generation of secondary seismic waves through coupling of theelectromagnetic source field to various rock formations (electroseismiceffect) and subsequent generations of secondary electromagnetic fieldsthrough coupling of the generated seismic waves with various rockformations (seismoelectric effect) to probe those formations and thefluids they contain. Alternatively or in addition, passive surveying maydetect the generation of secondary electromagnetic fields throughcoupling of a seismic source field to various rock formations(seismoelectric effect) and subsequent generations of secondary seismicwaves through coupling of the generated electromagnetic fields withvarious rock formations (electroseismic effect) to probe thoseformations and the fluids they contain. Generation of tertiary andhigher order electromagnetic fields and seismic waves can also resultfrom additional couplings as the fields propagate towards the surface ofthe earth.

Other surveying techniques such as controlled-source electroseismic orseismoelectric surveying typically reject signals generated by suchpassively-generated conversions as background noise. Utilizing theteachings of the present disclosure, however, electromagnetic andseismic signals generated by seismoelectric and electroseismicconversions in response to a passive source of energy may be detectedand processed using various data processing techniques to identifyproperties of the subsurface earth formation. For example, a generatedseismic signal may be identified by detecting the characteristic timelags or frequencies associated with the seismic travel time using atime-selective method and determining the depth of origin of the seismicsignal from said time selective method.

Electromagnetic and/or seismic signals generated as a result ofelectroseismic or seismoelectric conversions may be detected in anyappropriate manner. For example, various sensors may be utilized todetect one or more of an electromagnetic signal and a seismic signalthat are generated by a subsurface earth formation in response to apassive-source electromagnetic or seismic signal, wherein theelectromagnetic signal is generated by an electroseismic orseismoelectric conversion of the passive-source electromagnetic orseismic signal. In some embodiments, arrays of sensors may be utilized.Data processing may be utilized to process signals to facilitateidentification of one or more of the subsurface earth formationproperties discussed above.

Using these techniques, various properties of the subsurface earthformation may be identified. For example, processing the detected signalmay indicate the presence of fluids such as hydrocarbons and aqueousfluid such as potable water, fresh water, and brine water in thesubterranean formation. In some embodiments, the teachings of thepresent disclosure may be utilized to identify additional properties ofthe subsurface earth formation, including but not limited to theexistence of the subsurface earth formation, depth of the subsurfaceformation, porosity and/or fluid permeability of the subsurface earthformation, the composition of one or more fluids within the subsurfaceearth formation, a spatial extent of the subsurface earth formation, anorientation of the boundaries of the subsurface earth formation, andresistivity of the subsurface earth formation. Based on the identifiedproperties, models may be developed of the subsurface earth formation,including three-dimensional structural and time-dependent models. Inaddition or in the alternative, the techniques of the present disclosuremay be utilized to identify the presence of and/or migration of variouspollutants, gasses, flooding in hydrocarbon production, fault movement,aquifer depth, water use, the presence of and/or migration of magma, andhydrofracturing properties.

In some embodiments, passive survey data obtained and/or collected as aresult of passive surveying may be processed with geophysical surveydata obtained and/or collected using various other surveying techniques.Processing passive survey data and other available sources ofgeophysical survey data may provide various technical benefits. Forexample, such processing may allow additional information, more completeinformation, and/or confirmation of information regarding subsurfaceearth formations. Such processing may take advantage of particularstrengths of other survey methods to establish a baseline for comparisonand/or determine particular properties for which those methods arewell-suited. As a result, passive surveying techniques combined withother available surveying techniques may result in a more completeunderstanding of the subsurface formation than would otherwise have beenavailable if the individual techniques were used alone.

While specific advantages have been enumerated above, variousembodiments may include all, some, or none of the enumerated advantages.Embodiments of the present disclosure and its advantages are bestunderstood by referring to FIGS. 1 through 9, wherein like numeralsrefer to like and corresponding parts of the various drawings.

Example embodiment of the present disclosure may include passiveelectroseismic surveying. Example embodiments of passive electroseismicsurveying utilize naturally occurring electromagnetic fields (e.g., theearth's background electromagnetic field) and their interactions withsubsurface formations through electroseismic and/or seismoelectricconversions. Passive surveying uses sources of electrical power that arenot specifically generated for the surveying method. Electric fieldsoriginating at the surface of the earth penetrate deeply into thesubsurface where they interact with certain rock formations to generateseismic waves. These seismic waves propagate back to the Earth's surfacewhere they are detected with one or both of seismic or electromagneticsensors. Detecting the characteristic time lags or frequenciesassociated with the seismic travel time determines the depth of originof the seismic signal. The amplitude of the returning seismic signal maybe indicative of one or more properties of the subsurface formation,including, for example the fluid content of a subsurface formation.

Passive-source electroseismic surveying (PSES) may provide informationabout a subterranean formation that is not available from other methods.With seismic resolution, passive electroseismic surveying can yieldinformation about formation porosity, fluid permeability, fluidcomposition, resistivity, depth, and lateral extent of a fluid-bearingformation. Although PSES provides information about many usefulformation properties, it may not independently measure electrical andseismic properties. A complementary measure of seismic andelectromagnetic properties might yield additional useful information.

An understanding of the passive electromagnetic and seismic effects thatis useful in passive surveying begins with an understanding of theelectromagnetic field within the earth, at least a portion of which maycomprise the naturally occurring background electromagnetic field of theearth. The earth's naturally occurring electromagnetic field includes abroad spectrum of frequencies, from sub-hertz frequencies to tens ofthousands of hertz frequencies, having a broad coverage over the surfaceof the earth. This broad spectrum allows for a broad range ofpenetration depths from tens of meters to tens of kilometers. Thevarious electromagnetic frequencies in the earth may result from variousnatural events such as electromagnetic fluctuations in the ionosphereand/or naturally occurring electromagnetic discharges in the atmosphere(e.g., lightning).

The earth's electromagnetic field propagates as an electromagneticmodulation that, unlike an acoustic wave, travels at the speed of anelectromagnetic wave in the subsurface, which is less than the speed ofan electromagnetic wave in a vacuum or air. The electromagnetic wave maytypically travel in the subsurface of the earth at a speed of about onehundred times greater than the speed of propagation of an acoustic wavein the seismic frequency band of about 1-100 Hz. Due to the relativespeed of the electromagnetic wave when compared to the seismic signal,the travel time of the electromagnetic wave into the subsurface earthformation is generally neglected for purpose of processingelectroseismic and seismoelectric data.

Passive electromagnetic and passive seismic surveying make use of twoproperties of subsurface electromagnetic propagation neglected in manyother surveying methods. In the first instance, an electromagneticmodulation with an electric field perpendicular to the surface of theEarth attenuates weakly. Such waves of high frequency can propagate andinteract with formations from the surface to several km in depth and arenot used in magnetotellurics. The high-frequency character of thesemodulations permits measurement of the electromagnetic travel time fromthe surface to a formation of interest. In certain example embodiments,this transit time measurement might be used to accurately determine thedepth. In the second instance, an electromagnetic modulation enteringthe Earth's surface interacts with near-surface formations byelectroseismic conversion. Electroseismic conversion creates seismicwaves that propagate downward into the Earth and might reflect fromreservoir rock or other formations of interest. The transit time for theseismic wave to travel downward from the earth's surface and back from alayer of interest accurately determines the depth where the reflectionoccurs.

Electromagnetic modulations from the atmosphere impinge uniformly on theearth over large areas. The atmospheric field subsequently converts to auniform electric field in the Earth's subsurface. That field rotates toa vertical orientation and travels to substantial depth where it mightreflect from contrasts in resistivity or polarization. The feasibilityof passive electromagnetic surveying is enhanced by the uniform electricfield that creates plane wave propagation in the subsurface. It is knownthat plane waves travel to greater depth than waves generated at a pointor from a finite source of any shape.

In a similar fashion, passive seismic generation at the Earth's surfacecan be used to image the subsurface. Seismic waves might be generated atthe Earth's surface by several mechanisms. First, the atmosphericelectric field generates electroseismic conversions at the surface.Second, atmospheric disturbances, such as thunder, create pressurechanges at the surface that also create seismic responses. Third,anthropomorphic pressure and ground level noises are created by humanactivity such as trucks, trains, and machinery. All of these sources ofseismic energy in the Earth are potential sources for imaging thesubsurface by seismic reflection or refraction.

Plane waves of seismic origin will penetrate most deeply into thesubsurface. Then sources of seismic energy that are uniform over largedistances are most promising for subsurface imaging. Sources thatoriginate in the atmospheric electric field or in large scale pressurefluctuations, such as created by thunder, are most likely to be uniformand penetrate deeply.

A vertical electric field is attenuated slowly with depth andvertically-propagating seismic p-wave attenuation and scattering arealso attenuated slowly. Seismic attenuation calculations suggest thatfrequencies up to the kilo-Hertz range might be useful to depths ofthousands feet.

The systems and methods disclosed herein advantageously utilize signalsthat have heretofore been neglected and/or not detected. Magnetotelluricsurveying generally involves the use of the natural electromagneticfields that originate in the earth's atmosphere. In magnetotelluricsurveying, naturally-occurring electromagnetic fields propagate into thesubsurface where they encounter rock formations of differing electricalconductivity. When the electromagnetic fields contact a formation of lowconductivity, such as is typical of hydrocarbon reservoirs, theelectromagnetic field measured at the surface of the earth changes.Spatially-dependent electromagnetic fields measured on the earth'ssurface can be used to indicate the presence of low-conductivityformations that might contain hydrocarbons. Magnetotelluric surveyinghas several limitations. Only low-frequency, long-wavelengthelectromagnetic stimulation may reach prospective reservoirs because thehigh-frequency, horizontal electric fields are rapidly attenuated by theconducting earth. Long-wavelength electromagnetic waves limit thespatial resolution of magnetotellurics making reservoir delineationdifficult. Additionally, magnetotelluric surveying only providesinformation about formation electrical conductivity and does not yielddata revealing information about porosity, permeability, or reservoirstructure.

In contrast to magnetotelluric surveying methods, the passiveelectromagnetic surveying method makes use of the time it takes anelectromagnetic wave to travel from the surface to the target formationand the time it takes to return to the surface. The transit timemeasurement yields an accurate measure of the depth of a formation.Additionally, passive electromagnetic surveying uses the verticalcomponent of the passive electric field. It is known that the verticalcomponent of the electric field penetrates deeply into the earth atfrequencies that are higher than possible with magnetotellurics.

In general, active-source seismic surveying uses frequencies well below1000 Hz. A typical seismic survey for 3D imaging might be restricted tofrequencies below 200 Hz and more often to frequencies below 100 Hz. Therestriction on frequency is limited by several factors. First, theseismic sources couple poorly to the ground. High frequencies areattenuated at the source. Second, geophones used to detect seismicenergy couple poorly to the surface. Third, the seismic source andreceivers are point sources and receivers. Energy propagates rapidlyaway from point sources with geometric spreading. It is then difficultto detect frequencies above 100 Hz at target depths of thousands offeet.

Passive measurements of seismic energy can overcome the frequencylimitations of conventional seismic studies. In particular, passiveseismic measurements detect the Earth's electric field that then createsa seismic wave by electroseismic conversion in the near-surface. Anelectromagnetic detector is not limited to the ground coupling problemsassociated with seismic sources and sensors. Additionally, anelectromagnetic source in the atmosphere can consist of elements ofarbitrarily high frequencies. Acoustic sources such as thunder andanthropomorphic sources are less likely to have high frequencycomponents. These considerations mediate against using seismic sourcesand receivers to study passive seismology.

Example embodiments of passive seismic surveying use an electromagneticdetector of the same kind used in electroseismology. The significantdifference between the two measurements is that the passive seismicsignal is generated by reflections from subsurface boundaries, it doesnot involve electroseismic conversion at target depths, and it arrivesat twice a seismic travel time. That is, the seismic signal arrives atdouble the electroseismic arrival time.

In some example embodiments, a single sensor type, an electric ormagnetic field sensor, can be used to detect high-frequency, passive,seismic energy and high-frequency passive electromagnetic energy thatare useful as complementary measurements to passive electroseismology.

Passive electromagnetic and seismic surveying can be used alone or incombination to overcome limitations of present technologies forhydrocarbon exploration and production surveying. In the absence of highpower sources of electrical or seismic energy, the costs, environmental,and safety concerns are reduced. Example implementations of the methodsmay yield high spatial resolution of hydrocarbon or aquifer formations.Example implementations of the methods may provide a measure ofelectrical resistivity and seismic properties including velocities ofseismic and electromagnetic waves.

FIGS. 1 and 2 are perspective diagrams illustrating an example system 10for electroseismic and seismoelectric surveying. Example system 10includes electromagnetic sensors 26, seismic sensors 28, and computingsystem 30. FIG. 1 illustrates an embodiment in which system 10 isgenerally configured to utilize signals 14 propagated by a passiveelectromagnetic source 12 of electromagnetic energy to performgeophysical surveying. FIG. 2 illustrates an embodiment in which system10 is generally configured to utilize signals 20 and/or 22, which may bepropagated by a passive seismic source 40.

As illustrated in FIG. 1, sensors 26 and/or 28 generally detect signalsgenerated by subsurface earth formation 16 in response to aelectromagnetic signal 14 propagated from passive electromagnetic source12. Computing system 30 may then process detected signals using varioussignal processing techniques to identify properties and/or features ofsubsurface earth formation 16. System 10 may detect seismic signals 20generated due to the electroseismic interactions between theelectromagnetic signal 14 and the subsurface formation 16, either aloneor in combination with detecting electromagnetic signal 22, which may begenerated as a result of seismoelectric conversions of seismic signals20. One or more of the detected signals may then be processed todetermine one or more properties of the subsurface earth formation.

Passive electromagnetic source 12 represents any appropriate passivesource of electromagnetic energy. In certain example embodiments,passive electromagnetic source 12 may include the earth's naturalelectromagnetic field. In certain example embodiments, passiveelectromagnetic source 12 may include one or more man-made sources ofelectromagnetic or seismic energy that are generally not created for thepurpose of surveying of subterranean formations. The man made sources ofelectromagnetic energy for passive surveying may include electromagneticenergy from power lines or other sources of electromagnetic energy.Passive electromagnetic source 12 propagates electromagnetic energy intothe subsurface of the earth as electromagnetic signal 14.Electromagnetic signal 14 may represent, for example, an electromagneticplane wave 14. As electromagnetic signal 14 propagates into the earth,it may encounter various subsurface earth formations 16. The interactionof electromagnetic signal 14 and subsurface earth formation 16 may causean electroseismic conversion to take place at an edge and/or boundary 18of subsurface formation 16. As a result, one or more seismic waves 20may propagate towards the surface of the earth. Electromagnetic signal22 may be generated as a result of a seismoelectric conversion asseismic signals 20 a propagate towards the surface. Electromagneticsensors 26 may detect electromagnetic signals 22. Seismic sensors 28 maydetect seismic signals 20 b.

Passive electromagnetic source 12 may represent earth's naturallyoccurring electromagnetic field. Earth's naturally occurringelectromagnetic field may include a broad spectrum of frequencies, fromsub-hertz frequencies to tens of thousands of hertz frequencies, havinga broad coverage over the surface of the earth. This broad spectrumallows for a broad range of penetration depths of electromagnetic signal14 from tens of meters to tens of kilometers. This broad spectrumfurther may permit detection of subsurface structures with high spatialand depth resolution. The corresponding frequencies of electromagneticsignal 14 in the earth may result from variations in passiveelectromagnetic source 12 due to various natural events such aselectromagnetic fluctuations in the ionosphere, naturally occurringelectromagnetic discharges in the atmosphere such as lightning, and/orother electromagnetic events. In some embodiments, passiveelectromagnetic source 12 of electromagnetic signals 14 may includenatural sources of electromagnetic radiation, which may havesufficiently low frequencies to reach and interact with subterraneanformation 16. As another example, passive electromagnetic source 12 mayinclude power transmission lines, which may generate electromagneticsignals 14 of appropriate strength and/or frequency to interact withsubterranean formation 16.

Electromagnetic signal 14 represents an electromagnetic wave,electromagnetic plane wave, or other appropriate electromagnetic signalthat propagates into the Earth from passive electromagnetic source 12.For example, in response to Earth's electromagnetic field,electromagnetic signal 14 may propagate into the Earth as anelectromagnetic modulation that, unlike an acoustic wave, travels at thespeed of an electromagnetic wave in the subsurface. The speed of anelectromagnetic wave in the subsurface may generally be less than thespeed of an electromagnetic wave in a vacuum or air. Electromagneticsignal 14 may typically travel in the subsurface of the earth at a speedof about one hundred times greater than the speed of propagation of anacoustic wave in the seismic frequency band of about 1-100 Hz. Due tothe relative speed of electromagnetic signal 14 when compared to aseismic signal, the travel time of the electromagnetic signal 14 intothe subsurface earth formation may, in some embodiments, be ignored whenprocessing the detected electromagnetic field 22 and/or detected seismicsignals 20. Although illustrated as a static field, it should be notedthat electromagnetic signal 14 may be a time-varying field.

Electromagnetic signal 14 may propagate into the subsurface of the earthas an approximate plane wave modulation, including over subsurfaceformation 16 of interest. The term “plane wave” may refer to a wave withsubstantially uniform amplitude on a plane normal to a velocity vectorof electromagnetic signal 14. The velocity vector may be generallyvertical, although not necessarily perpendicular to the surface of theEarth above subsurface earth formation 16. For example, a velocityvector may be substantially vertical but may appear inclined relative toa vertical axis at the surface where the surface is on an incline, suchas on a hillside or other incline. As a result of the electroseismiceffect and/or seismoelectric effect, the seismic signals 20 and/orelectromagnetic signals 22 resulting from electromagnetic signals 14 maybe generated substantially uniformly across subsurface formation 16. Asa result, seismic signals 20 and/or electromagnetic signals 22 may eachform a substantially vertical plane wave traveling to the surface of theEarth.

Subsurface earth formation 16 represents any subsurface earth formationof interest for the purposes of geophysical surveying. Subsurface earthformation 16 may represent a geologic formation that holds one or morefluids. In some embodiments, subsurface earth formation 16 represents aporous rock formation able to hold fluids. A porous rock formation may,for example, include solid rock portion interspersed with channel-likeporous spaces. A porous rock formation may, for example, include anearth substance containing non-earthen volume or pore space, and mayinclude, but is not limited to, consolidated, poorly consolidated, orunconsolidated earthen materials. Fluids held by subsurface earthformation 16 may be hydrocarbons such as oil and gas, water (includingfresh, salt, potable, or briny water), helium, carbon dioxide, minerals,or other earth fluids. In some embodiments, subsurface earth formation16 may represent a formation holding pollutants, magma, or moltenmaterial. Subsurface earth formation 16 may represent a geologic layer,a stratographic trap, a fault, a fold-thrust belt, or other geographicformation of interest. Subsurface earth formation 16 may represent aprospective or potential area of interest for exploration operations,drilling operations, production enhancement operations, or fluidproduction.

Subsurface earth formation 16 may include a polarizable fluid includingone or more fluid dipoles 114 associated with a fluid in subsurfaceearth formation 16. As a result, an electrochemical interaction may formbetween the polarizable fluid and the solid rock portions at boundary18. The electrochemical interaction is represented by the “+” symbol inthe fluid portion and the “−” symbol in the solid rock portion.Electromagnetic signals 14 may encounter and/or interact with fluiddipoles 114 of subsurface earth formation 16. In particular, theelectromagnetic signals 14 may cause a change in the polarization ofdipoles 114 in the pore fluid, which in turn may cause a pressure pulse118 to be generated. For example, electromagnetic signals 14 may modifythe electrochemical bonds or move the charges of fluid dipoles 114,thereby effectively creating pressure pulse 118 where the interactionsare distorted. Pressure pulse 118 may represent a change in pressureand/or fluid flow that produces a time-varying pressure gradient, whichmay then propagate and/or be transmitted into the earth formation (orrock) at boundary 18 of subsurface earth formation 16. Electromagneticsignals 14 exist throughout the fluid area and may primarily affect thecharges of the dipoles 114 which are at or near boundary 18 of the rock.The pressure gradient produced by pressure pulse 118 may propagatetowards the surface as seismic signal 20. In should be noted that thesolid rock portion may have an existing natural surface charge over atleast a portion of the rock surface. The electrochemical interaction mayresult in a local pore fluid dipole 114 that causes a local backgroundelectromagnetic field. Moreover, the sign of the backgroundelectromagnetic field or field polarity direction depends on the surfacecharge on the solid and the way the fluid screens out that charge. Forexample, for clay layers, the charge is typically as shown asillustrated. In other materials such as carbonates, however, the chargemay be reversed. Thus, an appropriate subsurface formation 16 may be asubsurface source of seismic energy.

Boundary 18 may represent an appropriate edge, boundary, fluid surface,or interface between subsurface earth formation 16 and other portions ofthe subsurface. Boundary 18 may represent the boundary of a hydrocarbonreservoir, stratographic trap, fold thrust belt, geologic rock layer, orother geological formation holding or likely to hold fluids and otherminerals of interest. Boundary 18 may represent a boundary between anytwo types of subsurface materials.

Electroseismic energy conversion may occur at the boundary 18 betweentwo types of rock. For example, the electroseismic energy conversion mayoccur at the boundary 18 between reservoir rock and the sealing and/orconfining rock. Alternatively, electroseismic energy conversion mayoccur at an interface 18 between pore fluids, for example, between oiland water. At the rock and/or fluid interfaces 18 there may be agradient in the chemical potential. For example, at the boundary 18between a silicate rock and a carbonate rock, a chemical reaction mayoccur in the comingled pore fluids. For example, the silicate maydissolve the carbonate, and the silicate ions in solution may react withthe carbonate ions in solution. The overall reaction may be driven by agradient in the chemical potential at the interface 18. The reactionproduct between positive and negative ions in solution is electricallyneutral and may precipitate out of solution. When a precipitate isformed, the resulting deposition of the precipitate strengthens therock, increases its hardness, and increases the electrical resistivityof the interface. During the reactions in pore spaces, concentrationgradients of charged ions may be created within the pore fluids. Theseconcentration gradients may produce an electrochemical-potentialgradient which may manifest itself as a macroscopic electrical potentialgradient. The internal electrical potential gradients at the interfacesmay create internal stresses, and the interaction of the earth'sbackground electromagnetic field 14 with the electrochemical-potentialgradient may change these internal stresses. Due to the naturalmodulations in the earth's background electromagnetic field 14, theinternal stresses may be modulated, accounting for the nonlinearelectroseismic conversions that may be measured and used by system 10.

Seismic signals 20 represent any seismic signals and/or seismic wavesgenerated by the electroseismic effect in response to electromagneticsignal 14. As noted above, seismic signals 20 may represent asubstantially vertical plane wave that travels towards the surface ofthe Earth. Seismic signals 20 may generate subsequent secondaryelectromagnetic fields and seismic waves through various combinations ofthe electroseismic and seismoelectric effects as seismic signals 20propagate to the surface. For example, as illustrated, seismic wave 20 amay be converted by the seismoelectric effect to an electromagneticsignal 22 at a near surface formation 24. In some embodiments, seismicsignals 20 may represent secondary seismic signals generated as a resultof various seismoelectric and/or electroseismic conversions of seismicsignals 20 as they propagate towards the surface. Seismic signals 20 mayrepresent any mechanical seismic wave that propagates in the subsurfaceof the earth and may include, but is not limited to, P- and S-waves.

Electromagnetic signals 22 represent any electromagnetic signals,electromagnetic fields, or electromagnetic waves generated by theseismoelectric effect in response to seismic signals 20. As noted above,electromagnetic signals 22 may represent a substantially vertical planewave traveling to the surface of the Earth. Electromagnetic signals 22may generate subsequent secondary seismic signals and electromagneticsignals as electromagnetic signals 22 propagate to the surface.Electromagnetic signals 22 may represent secondary electromagneticsignals generated as a result of various seismoelectric and/orelectroseismic conversions of seismic signals 20 as they propagatetowards the surface. In some embodiments, electromagnetic signals 22 maybe detectable in the near-surface of the Earth and/or at some distanceabove the surface of the Earth. In addition, electromagnetic signals 22may represent a time-variant electromagnetic field resulting from theseismoelectric effect. Electromagnetic signals 22 may modulate anelectromagnetic field within the Earth, such as in the near surface 24and may thus be referred to as a modulating signal. “Modulation,” or“modulating,” may refer to frequency modulation, phase modulation,and/or amplitude modulation. For example, seismic signals 20 may travelto the near-surface 24 and directly modulate an electromagnetic fieldwithin the near-surface 24. Seismic signals 20 may cause a change in theelectrical impedance in near-surface 24, which may result in atime-dependent variation of electromagnetic signals 22 and/or thepassage of seismic signals 20 may interact with a fluid or rock boundaryat near surface 20 to produce electromagnetic signals 20.

Electroseismic conversions may also produce nonlinear electromagneticconversions. Seismoelectric and electroseismic effects generate harmonicresponses where the coupling of electromagnetic signals 22 and seismicsignals 20 create new modulations at frequencies that are harmonics ofthe electromagnetic signals 22 and seismic signals 20. Accordingly,electromagnetic signals 22 and seismic signals 20 may represent one ormore non-linear electromagnetic responses. Nonlinear electroseismicconversions may produce signals useful during processing. In someembodiments, nonlinear, harmonic signals having frequency components athigher frequency harmonics of the passive electromagnetic source 12'sfundamental frequency, such as those frequencies present in the earth'sbackground electromagnetic field, may be detected as a result ofdistortions of electromagnetic signals 14 interacting with subsurfaceearth formation 16 when it contains at least one fluid. The harmonicsignals may be processed alone or in conjunction with the fundamentalfrequencies of the seismic signals 20 and/or the electromagnetic signals22 to determine one or more properties of the subsurface earthformation. In some embodiments, system 10 may be utilized to detectand/or isolate the harmonic signals that may be present in bothelectromagnetic signals 22 and seismic signals 20.

Subsurface formation 16 may generate seismic signals 20 and/orelectromagnetic signals 22 particularly when fluid is present in aporous formation, such as formations of high permeability. Accordingly,seismic signals 20 and/or electromagnetic signals 22 may indicate thepresence of that fluid and/or may be utilized by system 10 to locateand/or potentially locate particular fluids, such as hydrocarbons,Helium, carbon dioxide, or water, water, or other types of fluids asdescribed above. In addition, when conventional seismic reflectionboundaries 18 exist between subsurface formation 16 and the surface,seismic reflections may occur and may be detected by seismic sensors 20.

Near-surface formation 24 represents a subsurface formation at or nearthe surface of the Earth. Near-surface formation 24 may, for example,represent a water table or other porous rock layer. Seismic signals 20may interact with fluid in pores of near-surface formation 24. As aresult, charges within the pore may be modified. The pore may, forexample, contain fresh water as is present in the water table. Theresulting modification of the charges may generate an alternatingcurrent field, which may lead to the emission of electromagnetic signals22 through the seismoelectric effect.

Electromagnetic sensors 26 represent any suitable combination of sensingelements capable of detecting and/or measuring at least some portion ofelectromagnetic signals 22. Electromagnetic sensors 26 may becommunicatively coupled to computing system 30 and/or configured tooutput detected signals to computing system 30. In some embodiments,sensors 26 may be configured to detect and/or isolate the verticalcomponent of the electromagnetic signals 22. As noted above,electromagnetic signals 22 may be emitted above the surface of the earthas a detectable electromagnetic field. It should also be noted that anelectromagnetic field generally includes an electric field and amagnetic field. Accordingly, electromagnetic sensor 26 may be capable ofdetecting electromagnetic signals 22, an electric portion ofelectromagnetic signals 22, and/or a magnetic portion of electromagneticsignals 22. In some embodiments, electromagnetic sensor 26 may representa magnetic field detector capable of detecting a magnetic field. In someembodiments, electromagnetic sensors 26 may be configured to attenuateand/or reject horizontal or vertical electromagnetic signals.

Electromagnetic sensors 26 may be arranged in an array and/or in avariety of patterns. Any appropriate number of electromagnetic sensors26 may be arranged in the array or pattern. For example, an array ofelectromagnetic sensors 26 may include anywhere from two to thousands ofsensors. In some embodiments, electromagnetic sensors 26 may represent aset of sensors that includes one or more magnetic field detectors, oneor more electric field detectors, and one or more electromagnetic fielddetectors, which may be used in particular locations for passivesurveying. The array may be configured of one or more disposedelectromagnetic sensors, such as sensor 26 a and 26 b, separated by anappropriate lateral distance. For example, sensor 26 a and 26 b may belocated anywhere between several inches to several miles apart.

Sensors 26 may comprise any type of sensor capable of measuring thevertical electric field component of electromagnetic signals 22 in thenear surface 24 of the Earth. In some embodiments, additional oralternative signals may also be measured including the backgroundvertical portion of electromagnetic signals 14, the passiveelectromagnetic source 12 of electromagnetic radiation, one or morecomponents of the magnetic field, one or more horizontal components ofthe electromagnetic signal and/or one or more components of the seismicamplitude. In some embodiments, one or more electromagnetic fielddetectors may be configured to measure a horizontal component of theearth's electromagnetic field in one or more dimensions. For example,sensors 26 may include electrode pairs disposed in a horizontalalignment to measure one or more horizontal components ofelectromagnetic signals 22 and/or electromagnetic signals 14. In someembodiments, sensor 26 may be configured to measure multiple componentsof electromagnetic signals 22 and/or 14. For example, sensor 26 mayrepresent a two-axis electromagnetic field detector and/or a three-axiselectromagnetic field detector.

Sensors 26 may be disposed above the surface of the Earth and/or withinthe Earth. In some embodiments, sensor 26 may be placed at or on thesurface of the Earth or at any distance above the surface of the Earth.For example, electromagnetic sensors 26 may be disposed anywhere fromone to one hundred feet above the Earth, depending on the relativeamplification capabilities of sensors 26 and the attenuation ofelectromagnetic signals 22. Sensors 26 may also be placed in aircraft.In certain example embodiments, the aircraft fly low. In someembodiments, sensors 26 may be disposed above and/or below the watertable, above and/or below subsurface earth formation 16, and/or anyappropriate combinations of locations and depths. Sensors 26 may bemaintained in one location during a detection period of particularelectromagnetic signals 22 and/or may be subsequently moved to provideanother detection period. Additionally or alternatively, a plurality ofsensors 26, such as an array, may be used to provide multiplesimultaneous measurements at multiple locations. For example,electromagnetic sensors 26 may be disposed within a wellbore.Alternatively or in addition, an array of electromagnetic sensors 26 maybe disposed in the area above and/or surrounding the wellbore tofacilitate drilling operations and/or exploration of drilled fields. Amore detailed discussion of an example operation of such embodiments isdiscussed below with respect to FIG. 7. More detailed examples ofsensors 26 are illustrated in FIGS. 2A, 2B, and 2C.

Seismic sensors 28 represent any suitable combination of sensingelements capable of detecting and/or measuring at least some portion ofseismic signals 20. For example, sensors 28 may be configured to detectthe vertical component of seismic signals 20. Seismic sensors 28 may becommunicatively coupled to computing system 30 and/or configured tooutput detected signals to computing system 30. Seismic sensors 28 mayinclude, but are not limited to, geophones, hydrophones, and/oraccelerometers, including digital accelerometers. Sensors 28 mayrepresent a single-component geophone, a two-component geophone, or athree-component geophone. Sensors 28 may also represent a single-axisaccelerometer, a two-axis accelerometer, or a three-axis accelerometer.In some embodiments, seismic sensors 28 may represent one or morethree-component accelerometers. Additionally or alternatively, sensors28 may represent any appropriate combinations of these types of seismicsensors. For example, multiple types of sensors 28 may be utilized bysystem 10 to detect seismic signals 20. Seismic sensors 28 may measure aseismic wave in multiple directions, for example in one or twodirections parallel to the surface of the earth, in a directionperpendicular to the surface of the earth, and/or in a verticaldirection. Seismic sensors 28 may measure rotational seismic energywhere earth motion is circular around a horizontal or vertical axis.Rotational sensors may advantageously be used to identify surfaceseismic waves.

Seismic sensors 28 may be arranged in an array and/or in a variety ofpatterns. For example, seismic sensors 28 may be arranged and/or locatedin similar manners and locations as discussed above with respect tosensors 26. Any appropriate number of seismic sensors 28 may be arrangedin the array or pattern. As another example, a grid pattern may be used.Seismic sensors 28 may be laterally spaced apart by a distance relatedto the wavelength of the highest frequency surface seismic wavesexpected to be detected. That may include higher frequencies than thoseexpected to be produced by the electroseismic effect within thesubsurface earth formation. Seismic sensors 28 may be configured toattenuate and/or reject surface and/or horizontal seismic signals. Suchsignals may be caused by various sources including heavy equipment,vehicular traffic, and/or natural sources such as earthquakes and/orthunder.

In some embodiments, a pattern and/or array of electromagnetic sensors26 may overlap with a pattern or array of seismic sensors 28. Signalsdetected by sensors 26 and/or 28 may be transmitted to computing system30. In some embodiments, the signals may be suitably recorded, forexample, using a conventional seismic field recorder. Additionally oralternatively, each sensor may have its own recording device, and eachrecording device may be internal or external to the seismic sensor. Itshould be noted that while illustrated as including sensors 26 and 28,system 10 may include only sensors 26 or only sensors 28 as appropriatefor particular embodiments. Accordingly, any appropriate combination ofsensors 26 and/or sensors 28 may be utilized.

Sensors 26 and/or 28 may be placed in a wellbore. For example, in someimplementations one or more contacts are provided down-hole on theinside of a casing to measure electric potential. With such aconfiguration, the arrival of an electroseismic signal may be measuredfor locations along the wellbore. In other implementations, one or moreseismic sensors 28 may be positioned down-hole.

Sensors 26 and/or 28 may form all or a portion of a long-terminstallation, which may be utilized for long-term passive surveying.Signals 20 and/or 22 may be detected at multiple times over a period oftime, which may be periods of days, weeks, months, or years. Long-termsurveys may provide a time-based indication of various properties ofsubsurface earth formation 16, including any changes in the formationover the time period in which the signals are detected. System 10 maythus be used to monitor the development and/or depletion of ahydrocarbon field and/or water well or aquifer over periods ofproduction.

Computing system 30 represents any suitable combination of hardware,software, signal processors, and controlling logic to process, store,and/or analyze electromagnetic signals 22 and/or seismic signals 20received from sensors 26 and/or 28. Computing system 30 may include oneor more processors, memory, and/or interfaces. Computing system 30 may,for example, include an interface operable to communicatively couplewith and/or receive information from sensors 26 and/or 28. Computingsystem may be operable to receive and/or process passive survey datafrom sensors 26 and 28. Passive survey data may include, for example,data representative of signals 20 and/or 22. Computing system 30 mayinclude one or more appropriate analog-to-digital converters to digitizesignals 20 and/or 22 for digital signal processing. Alternatively or inaddition, sensors 26 and/or 28 may include appropriate analog-to-digitalconverters. Computing system 30 may include a recording and/or storagedevice operable to receive and store data received from sensors 26 and28. Computing system 30 may include, for example, digital and/or analogrecording devices and/or non-transitory media. In some embodiments,computing system 30 may be capable of processing detected seismic signal20 and the detected electromagnetic signal 22 in real-time without firstrecording the signals on a non-transitory medium.

Computing system 30 may form all or a portion of a recording vehicle, ahousing structure, or a weather resistant enclosure located proximatesensors 26 and/or 28. In some embodiments, computing system 30 may be atleast partially enclosed in a weather-resistant enclosure. Accordingly,computing system 30 may be capable of recording passive survey data overdays to weeks without human intervention. Moreover, while illustrated asexternal to sensors 26 and/or 28, computing system 30 may be internal orexternal to a housing of one or more sensors 26 and/or 28. Moreover,computing device 30 may be one of a plurality of computing devices 30used to record one or more electric and/or seismic signals. Computingdevice 30 may be capable of communicating with other computing devices30 or other data processing servers over a network (not illustrated).The network may be a wired or wireless communications network. Thus, anyof the data processing techniques described herein may be performed byone or more computing devices 30 and/or may be performed by a remotedata processing server, which may be capable of processing andcorrelating data from various computing devices 30. An exampleembodiment of computing system 30 is discussed in more detail below withrespect to FIG. 4.

As illustrated in FIG. 2, passive seismic source 40 represents anyappropriate passive source of seismic energy. For example, passivesource 40 may represent the earth's natural seismic energy. Passivesource 40 propagates seismic energy into the subsurface of the earth asseismic signal 42. Seismic signal 42 may represent, for example, aseismic plane wave 42. As seismic signal 42 propagates into the earth,it may encounter various subsurface earth formations 16. The interactionof seismic signal 42 and subsurface earth formation 16 may cause aseismoelectric conversion to take place at an edge and/or boundary 18 ofsubsurface formation 16. As a result, one or more electromagneticsignals 22 and/or seismic signals 20 may propagate towards the surfaceof the earth. Electromagnetic signal 22 may be generated as a result ofa seismoelectric conversion as seismic signals 20 propagate towards thesurface. Electromagnetic sensors 26 may detect electromagnetic signals22. Seismic sensors 28 may detect seismic signals 20. In someembodiments, seismic sensors 28 may detect seismic signals 40, which maybe used as a reference to detect a modulation of signals 20 and/or 22 bysubsurface earth formation 16.

Passive seismic source 40 may represent earth's naturally occurringseismic energy. Earth's naturally occurring seismic energy may include abroad spectrum of frequencies, from sub-hertz frequencies to tens ofthousands of hertz frequencies, having a broad coverage over the surfaceof the earth. This broad spectrum allows for a broad range ofpenetration depths of seismic signal 42 from tens of meters to tens ofkilometers. This broad spectrum further may permit detection ofsubsurface structures with high spatial and depth resolution. Thecorresponding frequencies of seismic signal 42 in the earth may resultfrom variations in passive source 40 due to various natural events suchas Earth quakes, tides, tectonic events, volcano activity, thunder, andatmospheric pressure fluctuations. In some embodiments, passive source40 of seismic signals 42 may include cultural sources of seismic waves,which may have sufficiently low frequencies to reach and interact withsubterranean formation 16. As another example, passive source 40 mayinclude well-drilling activities, pumping fluids, automobile noise,compressor noise, farming noise, and manufacturing noise, which maygenerate seismic signals 42 of appropriate strength and/or frequency tointeract with subterranean formation 16.

FIG. 2 includes several examples of passive seismic source 40, includingpassive seismic sources 40 a-40 e. Passive seismic source 40 a mayrepresent a source of seismic energy resulting from a drillingoperation. Passive seismic source 40 a may represent a localizeddrilling event at a particular depth (such as, for example, the head ofa drill bit or drilling apparatus interacting with the subsurface)and/or may represent vibrations from drilling activities along a lengthof the hole and/or casing. Passive seismic source 40 b may represent asource of seismic energy resulting from horizontal drilling activitiessuch as fracturing, hydrofracturing, or other drilling operations.Additionally or alternatively, passive seismic source 40 b may representseismic energy caused by fluid moving through rock pore spaces (whichmay be the result of hydrofracturing). Passive seismic sources 40 c and40 d may represent sources of seismic energy resulting from the Earth'snatural seismic activity and/or a microseismic or other natural event,as described above. Passive seismic source 40 b may represent a sourceof seismic energy resulting from a near-surface or surface event.Accordingly, passive seismic source 40 may include any appropriatesource of seismic energy and/or may be located in any appropriaterelationship to subsurface earth formation 16, including above, below,beside, or in subsurface earth formation 16. Additionally oralternatively passive seismic source 40 may include seismic energycaused by a drill bit, fracturing rock, fluid moving through rock porespaces, wells where drilling or pumping activity occurs, and/or bypollutant fluids migrating through the subsurface.

Seismic signal 42 represents a seismic wave, seismic plane wave, orother appropriate seismic signal that propagates into the Earth frompassive source 40. Accordingly, seismic signal 42 may emanate from anyappropriate passive seismic source 40, including those originating atthe Earth's surface and/or located at some appropriate depth below thesurface. For example, seismic signals 42 a-42 e may respectivelyoriginate from passive seismic sources 40 a-40 e. It should beunderstood that the various signals illustrated in FIGS. 1 and 2 aredepicted in different figures for the sake of clarity only. Accordingly,particular embodiments of system 10 may be capable of utilizing signals20 and/or 22 propagated by passive electromagnetic source 12 and/orpassive seismic source 40. Moreover, system 10 may be configured toutilize signals 20 and/or 22 from passive electromagnetic source 12 atparticular times while utilizing signals 20 and/or 22 from passiveseismic source 40 at particular other times and/or may utilize thesignals at the same time. For example, passiveelectroseismic/seismoelectric surveying utilizing passive seismicsources 40 and/or passive electromagnetic sources 12 may be collectedduring drilling or fracturing or enhanced oil recovery to acquireinformation about hydrocarbons and/or other fluids. Survey data frompassive electromagnetic sources 12 may be collected, for instance, whenpassive seismic sources 40 are attenuated. For example, the drillingoperation may be paused and/or finished. As another example, computingsystem 30 may perform passive surveying during drilling, fracturing,and/or enhanced oil recovery to acquire information about hydrocarbonsand/or other fluids.

In operation, system 10 detects, stores, and/or analyzes electromagneticsignals 22 and/or seismic signals 20. Sensors 26 and 28 respectively maydetect electromagnetic signals 22 and seismic signals 20. Each sensormay transmit the detected signals to computing device 30 for storageand/or processing. Computing device 30 may record the resultingelectromagnetic signals 22 and/or seismic signals 20. Computing device30 may process electromagnetic signals 22 and/or seismic signals 20 toidentify various properties associated with subsurface formation 16.Sensors 26 and/or 28 may additionally or alternatively detect signalsgenerated by subsurface earth formation 16 in response to aelectromagnetic signal 42 propagated from passive seismic source 40.Computing system 30 may then process detected signals using varioussignal processing techniques to identify properties and/or features ofsubsurface earth formation 16. Thus, the techniques discussed in thepresent disclosure may be utilized to analyze signals 20 and/or 22generated as a result of passive electromagnetic source 12 and/orpassive seismic source 40. Certain examples of the operation of system10 provided below may be discussed with respect to a passiveelectromagnetic source 12, but it should be noted that the teachings ofthe present disclosure apply similarly and/or the same to signalsgenerated by passive seismic source 40.

Certain embodiments of system 10 monitor one or more drillingoperations, production enhancement operations (e.g., fracturing), orfluid production operations by processing signals from a set of sensors,including one or more electromagnetic sensors 26 and one or more seismicsensors 28. Sensors may include but are not restricted to: geophonesthat may detect seismic and/or electromagnetic signals; accelerometersthat may or may not include an electromagnetic sensor; a capacitiveelectric field sensor that may or may not include a seismicaccelerometer and or a geophone and or a magnetic field sensor; a coilelectromagnetic sensor that may or may not include a geophone oraccelerometer for seismic detection and may or may not include amagnetic field sensor; an electromagnetic field antenna that may or maynot include a seismic sensor and may or may not include an electricfield and/or a magnetic field sensor, said antenna being a dipoleantenna, a monopole antenna or other electromagnetic field antenna aswell-known to those skilled in the art; a magnetic sensor that may ormay not include an electric field sensor and may or may not include aseismic sensor.

FIG. 4 illustrates an example computer system 30 suitable forimplementing one or more embodiments disclosed herein. The computersystem 30 includes a processor 482 (which may be referred to as acentral processor unit or CPU) that is in communication with memorydevices including secondary storage 484, read only memory (ROM) 486,random access memory (RAM) 488, input/output (I/O) devices 490, andnetwork connectivity devices 492. The processor may be implemented asone or more CPU chips.

It is understood that by programming and/or loading executableinstructions onto the computing system 30, at least one of the CPU 482,the RAM 488, and the ROM 486 are changed, transforming the computingsystem 30 in part into a particular machine or apparatus having thenovel functionality taught by the present disclosure. It is fundamentalto the electrical engineering and software engineering arts thatfunctionality that can be implemented by loading executable softwareinto a computer can be converted to a hardware implementation by wellknown design rules. Decisions between implementing a concept in softwareversus hardware typically hinge on considerations of stability of thedesign and numbers of units to be produced rather than any issuesinvolved in translating from the software domain to the hardware domain.Generally, a design that is still subject to frequent change may bepreferred to be implemented in software, because re-spinning a hardwareimplementation is more expensive than re-spinning a software design.Generally, a design that is stable that will be produced in large volumemay be preferred to be implemented in hardware, for example in anapplication specific integrated circuit (ASIC), because for largeproduction runs the hardware implementation may be less expensive thanthe software implementation. Often a design may be developed and testedin a software form and later transformed, by well known design rules, toan equivalent hardware implementation in an application specificintegrated circuit that hardwires the instructions of the software. Inthe same manner as a machine controlled by a new ASIC is a particularmachine or apparatus, likewise a computer that has been programmedand/or loaded with executable instructions may be viewed as a particularmachine or apparatus.

The secondary storage 484 is typically comprised of one or more diskdrives or tape drives and is used for non-volatile storage of data andas an over-flow data storage device if RAM 488 is not large enough tohold all working data. Secondary storage 484 may be used to storeprograms which are loaded into RAM 488 when such programs are selectedfor execution. The ROM 486 is used to store instructions and perhapsdata which are read during program execution. ROM 486 is a non-volatilememory device which typically has a small memory capacity relative tothe larger memory capacity of secondary storage 484. The RAM 488 is usedto store volatile data and perhaps to store instructions. Access to bothROM 486 and RAM 488 is typically faster than to secondary storage 484.The secondary storage 484, the RAM 488, and/or the ROM 486 may bereferred to in some contexts as computer readable storage media and/ornon-transitory computer readable media.

I/O devices 490 may include printers, video monitors, liquid crystaldisplays (LCDs), touch screen displays, keyboards, keypads, switches,dials, mice, track balls, voice recognizers, card readers, paper tapereaders, or other well-known input devices.

The network connectivity devices 492 may take the form of modems, modembanks, Ethernet cards, universal serial bus (USB) interface cards,serial interfaces, token ring cards, fiber distributed data interface(FDDI) cards, wireless local area network (WLAN) cards, radiotransceiver cards such as code division multiple access (CDMA), globalsystem for mobile communications (GSM), long-term evolution (LTE),worldwide interoperability for microwave access (WiMAX), and/or otherair interface protocol radio transceiver cards, and other well-knownnetwork devices. These network connectivity devices 492 may enable theprocessor 482 to communicate with the Internet or one or more intranets.With such a network connection, it is contemplated that the processor482 might receive information from the network, or might outputinformation to the network in the course of performing theabove-described method steps. Such information, which is oftenrepresented as a sequence of instructions to be executed using processor482, may be received from and outputted to the network, for example, inthe form of a computer data signal embodied in a carrier wave.

Such information, which may include data or instructions to be executedusing processor 482 for example, may be received from and outputted tothe network, for example, in the form of a computer data baseband signalor signal embodied in a carrier wave. The baseband signal or signalembodied in the carrier wave generated by the network connectivitydevices 492 may propagate in or on the surface of electrical conductors,in coaxial cables, in waveguides, in an optical conduit, for example anoptical fiber, or in the air or free space. The information contained inthe baseband signal or signal embedded in the carrier wave may beordered according to different sequences, as may be desirable for eitherprocessing or generating the information or transmitting or receivingthe information. The baseband signal or signal embedded in the carrierwave, or other types of signals currently used or hereafter developed,may be generated according to several methods well known to one skilledin the art. The baseband signal and/or signal embedded in the carrierwave may be referred to in some contexts as a transitory signal.

The processor 482 executes instructions, codes, computer programs,scripts which it accesses from hard disk, floppy disk, optical disk(these various disk based systems may all be considered secondarystorage 484), ROM 486, RAM 488, or the network connectivity devices 492.While only one processor 482 is shown, multiple processors may bepresent. Thus, while instructions may be discussed as executed by aprocessor, the instructions may be executed simultaneously, serially, orotherwise executed by one or multiple processors. Instructions, codes,computer programs, scripts, and/or data that may be accessed from thesecondary storage 484, for example, hard drives, floppy disks, opticaldisks, and/or other device, the ROM 486, and/or the RAM 488 may bereferred to in some contexts as non-transitory instructions and/ornon-transitory information.

In some embodiments, computing system 30 may comprise two or morecomputers in communication with each other that collaborate to perform atask. For example, but not by way of limitation, an application may bepartitioned in such a way as to permit concurrent and/or parallelprocessing of the instructions of the application. Alternatively, thedata processed by the application may be partitioned in such a way as topermit concurrent and/or parallel processing of different portions of adata set by the two or more computers. In some embodiments,virtualization software may be employed by the computing system 30 toprovide the functionality of a number of servers that is not directlybound to the number of computers in the computing system 30. Forexample, virtualization software may provide twenty virtual servers onfour physical computers. In some embodiments, the functionalitydisclosed above may be provided by executing the application and/orapplications in a cloud computing environment. Cloud computing maycomprise providing computing services via a network connection usingdynamically scalable computing resources. Cloud computing may besupported, at least in part, by virtualization software. A cloudcomputing environment may be established by an enterprise and/or may behired on an as-needed basis from a third party provider. Some cloudcomputing environments may comprise cloud computing resources owned andoperated by the enterprise as well as cloud computing resources hiredand/or leased from a third party provider.

In some embodiments, some or all of the functionality disclosed abovemay be provided as a computer program product. The computer programproduct may comprise one or more computer readable storage medium havingcomputer usable program code embodied therein to implement thefunctionality disclosed above. The computer program product may comprisedata structures, executable instructions, and other computer usableprogram code. The computer program product may be embodied in removablecomputer storage media and/or non-removable computer storage media. Theremovable computer readable storage medium may comprise, withoutlimitation, a paper tape, a magnetic tape, magnetic disk, an opticaldisk, a solid state memory chip, for example analog magnetic tape,compact disk read only memory (CD-ROM) disks, floppy disks, jump drives,digital cards, multimedia cards, and others. The computer programproduct may be suitable for loading, by the computing system 30, atleast portions of the contents of the computer program product to thesecondary storage 484, to the ROM 486, to the RAM 488, and/or to othernon-volatile memory and volatile memory of the computing system 30. Theprocessor 482 may process the executable instructions and/or datastructures in part by directly accessing the computer program product,for example by reading from a CD-ROM disk inserted into a disk driveperipheral of the computing system 30. Alternatively, the processor 482may process the executable instructions and/or data structures byremotely accessing the computer program product, for example bydownloading the executable instructions and/or data structures from aremote server through the network connectivity devices 492. The computerprogram product may comprise instructions that promote the loadingand/or copying of data, data structures, files, and/or executableinstructions to the secondary storage 484, to the ROM 486, to the RAM488, and/or to other non-volatile memory and volatile memory of thecomputing system 30.

In some contexts, a baseband signal and/or a signal embodied in acarrier wave may be referred to as a transitory signal. In somecontexts, the secondary storage 484, the ROM 486, and the RAM 488 may bereferred to as a non-transitory computer readable medium or a computerreadable storage media. A dynamic RAM embodiment of the RAM 488,likewise, may be referred to as a non-transitory computer readablemedium in that while the dynamic RAM receives electrical power and isoperated in accordance with its design, for example during a period oftime during which the computer 980 is turned on and operational, thedynamic RAM stores information that is written to it. Similarly, theprocessor 482 may comprise an internal RAM, an internal ROM, a cachememory, and/or other internal non-transitory storage blocks, sections,or components that may be referred to in some contexts as non-transitorycomputer readable media or computer readable storage media.

Example Electromagnetic Sensors and Sensor Arrangements

FIGS. 15A, 15B, and 15C are block diagrams illustrating example sensors26 for passive electroseismic and seismoelectric surveying. Asillustrated in the FIG. 15A, sensor 1560 may be a particular embodimentof sensor 26 that includes one or more conductive elements 1502 and1504, coupling network 1510, amplifier 1508, and signal processing unit1509. Sensor 1560 may be capable of detecting electroseismic signals 22,as previously discussed above with respect to sensor 26. Sensor 1560 mayoutput a signal representing detected electromagnetic signals 22. Sensor1560 may be installed and/or disposed in any appropriate housing,including weather-resistant housing, movable vehicles, and/or permanentinstallations, as is discussed above with respect to sensor 26. Sensor1560 generally operates by comparing a stable reference voltage to avoltage measurement responsive to electromagnetic signals radiated fromthe ground. Accordingly, sensor 1560 may be configured to sensevariations in the ground signal, which may be wholly or partiallycomprised of electromagnetic signals 22, as compared to a referencevoltage.

Conductive elements 1502 and 1504 are generally capable of measuringelectromagnetic signals radiated from the ground. As illustratedconductive element 1502 measures a stable reference voltage, whileconductive element 1504 is generally capable of measuring the verticalcomponent of electromagnetic signals 22. Conductive elements 1502, 1504may represent any appropriate capacitive and/or conductive plates orother sensing elements. As illustrated, conductive elements 1502 and1504 are capacitive plates that are arranged parallel to the surface ofthe Earth. A generally parallel arrangement to the surface of the Earthmay allow conductive element 1504 to respond to and/or measure thevertical component of electromagnetic signals 22, which may represent avertical electric field. Similarly, conductive element 1502 may beshielded from and/or configured not to measure the vertical component ofelectromagnetic signals 22. In some embodiments, conductive elements1502, 1504 may form a capacitor. Conductive elements 1502, 1504 may be aconductive metal such as copper, aluminum, or stainless steel.Particular embodiments of conductive elements 1502, 1504 may have anarea of several square inches to about several square feet. Asillustrated, conductive elements 1502, 1504 may be separated from theEarth by a distance x. Distance x may be any appropriate distance inwhich conductive elements 1502, 1504 may be capable of responding toelectromagnetic signals 22 transmitted into the air as a verticalelectric field. Conductive elements 1502, 1504 may be configuredrelatively close to the ground. For example, capacitive plates 1502,1504 may be separated from the Earth by about 10-12 inches in particularembodiments. It should be noted, however, that while particulardistances are discussed as example, any distance may be used in whichconductive elements 1502, 1504 are capable of detecting electromagneticsignals 22. Conductive elements 1502, 1504 may each be connected toinputs of amplifier 1508. Conductive element 1502 or conductive element1504 may also be connected to ground. It should be understood, however,that while a particular embodiment of conductive elements 1502 and 1504is discussed herein, any appropriate conductive elements may be used.For example, conductive element 1502 may represent a flat conductiveplate disposed next to conductive element 1504, which may be an antenna.Appropriate antennas may include flat conductive plates at predeterminedand/or fixed distances from the ground, concave conductive plates abovethe ground, multiple conductive plates with geometry to concentrate thesignal, metal screen or grid of wire in any appropriate shape and/orgeometry, monopole wire extending upwards from the ground, wire loopedaround a ferrite or steel core, or any other appropriate structurecapable of being used as an antenna. Moreover, conductive elements 1502and 1504 may represent any appropriate conductive elements arranged withgeometry to maximize self-capacitance. Also, while illustrated as twocomponents conductive elements 1502 and 1504 may be implemented as asingle component. For example, conductive elements 1502 and 1504 may beimplemented using a monopole wire extending upward from the groundand/or a battery arrangement. In some embodiments, conductive elements1502 and/or 1504 may represent a conductive sphere.

Amplifier 1508 represents any appropriate amplification circuit operableto compare signals generated by capacitive plate 1504 to referencesignals generated by capacitive plate 1502. Amplifier 1508 may, forexample, represent an operational amplifier. In some embodiments,amplifier 1508 may include any appropriate signal conditioning circuitsand/or components. For example, amplifier 1508 may be capable ofperforming any one or more of the pre-processing and/or processing stepsdiscussed above with respect to FIG. 1. Amplifier 1508 may includeappropriate inputs and outputs. As illustrated, capacitive plates 1502,1504 are connected to the inputs. The output may be connected tocomputing system 30. For example, amplifier 1508 may be capable ofoutputting detected electromagnetic signals 22 to computing system 30.Amplifier 1508 may, in some embodiments, include appropriateanalog-to-digital converters for digitizing detected electromagneticsignals 22.

Signal processing unit 1509 represents any appropriate combination ofhardware, software, and other components operable to process the outputof amplifier 1508. For example, signal processing unit 1509 may becapable of implementing any one or more of the pre-processing stepsdiscussed herein. Signal processing unit 1509 may behardware-implemented portion of sensor 1560 and/or may form a portion ofcomputing system 30. Signal processing unit 1509 may include one or morenotch filters, low pass filters, high pass filters, clamping circuits,sample and hold circuits, or any other appropriate signal conditioningcircuits.

Coupling network 1510 represents any appropriate network of componentsoperable to couple conductive elements 1502, 1504 to amplifier 1508. Asillustrated, coupling network 1510 includes a capacitor C1, inductor L1,capacitor C2 and a resistor R arranged as a pi filter. The pi filtergenerally is operable to select a desired frequency band for amplifier1508 and to exclude frequencies that may otherwise saturate amplifier1508. The resistor may be any appropriate resistance, and in someembodiments may be selected to set the time constant of the inputcircuitry of electromagnetic signals 22. Resistor R may be connectedacross the inputs to amplifier 1508 in parallel. Moreover, while aparticular embodiment of coupling network 1510 is illustrated, anyappropriate network components may be used. For example, couplingnetwork 1510 may include a matching resistor, a pi filter, atransformer, a resonant network, or any combination and number of thesecomponents.

Shielding 1512 represents any suitable electromagnetic shielding.Shielding 1512 may be configured to attenuate and/or prevent horizontalcomponents of electromagnetic fields from reaching conducting element1514. Shielding 1512 may be configured to surround all or a portion ofconductive elements 1502 and 1504. For example, as illustrated,shielding 1512 may comprise a structure that surrounds the top and sidesof conductive elements 1502 and 1504. Shielding 1512 may, for instance,be a cylindrical structure disposed vertically and that may be closed onat least one end, such as the top end. Alternatively, shielding 1512 mayrepresent a box or other appropriate enclosure. Shielding 1512 may bemade of any appropriate material operable to attenuate and/or preventelectromagnetic signals from propagating through the material. Forexample, shielding 1512 may be made of mu-metal, conductive plates orfoil, wire mesh, aluminized Mylar, insulating plates with suppliedstatic charge, and/or conductive plastic. Mu-metal may refer to one ormore classes of nickel-iron alloys that are characterized by ahigh-magnetic permeability. Shielding 1512 may shield against static orslowly varying electromagnetic fields that may otherwise interfere withthe detection of electromagnetic signals 22. Shielding 1512 may beelectrically connected and/or coupled to an input to amplifier 1508. Itshould also be understood that in particular embodiments, shielding 1512may or may not be appropriate and/or necessary.

In operation, electromagnetic signals 22 may be a time varying, verticalelectric field. The interaction of electromagnetic signals 22 withcapacitive plate 1504 may produce a charge on conductive elements 1504.The other plate 1502 may be shielded from electromagnetic signals 22.Accordingly, signals generate by plate 1502 may be interpreted as thereference voltage. Accordingly, a capacitive charge across conductiveelements 1502 and 1504 may result that corresponds to electromagneticsignals 22. In some embodiments, a resistor may be coupled in serieswith the charged conductive element 1502. At appropriate times, thecharged conductive plate 1502 may be discharged and thereby allow atime-varying field representative of electromagnetic signals 22 to bemeasured, processed, and/or recorded by computing system 30. By usingparallel conductive elements 1502, 1504, sensor 1560 may detect only thevertical components of electromagnetic signals 22 or otherelectromagnetic signals. Accordingly, the parallel plate design may beconfigured not to respond to the horizontal components ofelectromagnetic signals 22. While two conductive elements 1502, 1504 areshown, sensor 1560 may include a single plate appropriately groundedthrough one or more resistive devices and coupled to computing system30.

FIG. 15B illustrates sensor 1562, which may be a particular embodimentof sensor 26 that includes coupling network 1511, shielding 1512,conductive element 1514, electrode 1516, amplifier 1518, and signalprocessing unit 1519. Like sensor 1560, sensor 1562 may be capable ofdetecting electroseismic signals 22, as previously discussed above withrespect to sensor 26. Sensor 1560 may also output a signal representingdetected electromagnetic signals 22. Sensor 1560 may be installed and/ordisposed in any appropriate housing, including weather-resistanthousing, movable vehicles, and/or permanent installations, as isdiscussed above with respect to sensor 26.

Coupling network 1511 represents any appropriate network of componentsoperable to couple conductive elements 1502, 1504 to amplifier 1508. Asillustrated, coupling network includes a resistor R of an appropriateresistance, which may be selected to set the time constant of the inputcircuitry of electromagnetic signals 22. Resistor R may be connectedacross the inputs to amplifier 1508 in parallel. Moreover, while aparticular embodiment of coupling network 1511 is illustrated, anyappropriate network components may be used. For example, couplingnetwork 1511 may include a matching resistor, a pi filter, atransformer, a resonant network, or any combination and number of thesecomponents.

Shielding 1512 represents any suitable electromagnetic shielding, asdiscussed above with respect to FIG. 15A. Shielding 1512 may beconfigured to surround all or a portion of conducting element 1514. Forexample, as illustrated, shielding 1512 may comprise a structure thatsurrounds the top and sides of conducting element 1514. Shielding 1512may be electrically connected and/or coupled to an input to amplifier1518. As noted above, it should be understood that in particularembodiments, shielding 1512 may or may not be appropriate and/ornecessary.

Conductive element 1514 represents any appropriate conductive elementoperable to generate a stable reference signal shielded from one or morevertical and/or horizontal components of electromagnetic signals 22.Conductive element 1514 may represent a conductive plate. Asillustrated, conducting element 1514 is a conductive plate that includesmultiple folds that form multiple parallel portions of conductiveelement 1514. Folding conductive element 1514 into multiple foldedportions may allow conductive element 1514 to fit within a much smallervolume while also having a sufficiently large surface area to detectelectromagnetic signals 22. Additionally or alternatively, conductiveelement 1514 may include a conductive spine portion that forms abackbone or connection to multiple conductive fins. Conductive element1514 may be electrically connected and/or coupled to an input toamplifier 1518. Distance y represents any appropriate distanceseparating conductive element 1514 from the surface of the Earth. Forexample, in a particular embodiment, the distance may be about 24inches. In some embodiments, distance y may be relatively larger thandistance z.

Electrode 1516 represents any appropriate electrical componentconfigurable to form a connection with the Earth and/or detect one ormore vertical portions of electromagnetic signals 22. Electrode 1516 isconfigured to form an electrical contact with the Earth and may bedisposed within the Earth. For example, electrode 1516 may be disposedin a hole drilled into the Earth ranging from several inches to about 10feet to about 15 feet. Additionally or alternatively, electrode 1516 maybe disposed within the Earth at varying depths as needed to form anelectrical coupling with the Earth. In some embodiments, electrode 1516represents a porous pot electrode. Porous pot electrodes may include anappropriate salt and/or aqueous solution to form an electrical couplingwith the Earth. Suitable salts useful with the electrodes may include,but are not limited to, copper sulfate, silver chloride, cadmiumchloride, mercury chloride, lead chloride, and any combination thereof.In some embodiments, electrode 1516 may include a conductive electrodesuch as rods that are driven into the ground and/or sheets of metal,mesh sheets, and/or wires buried in trenches or in shallow pits.Electrode 1516 may be made of a variety of conductive materialsincluding, but not limited to, copper, stainless steel, aluminum, gold,galvanized metal, iron, lead, brass, graphite, steel, alloys thereof,and combinations thereof. Electrode 1516 may be electrically connectedand/or coupled to shielding 1512 and an input to amplifier 1518.Electrode 1516 may represent a porous pot, a conductive stake, a buriedlength of wire, a buried wire mesh, and/or a group of or combination ofthe aforementioned components.

Amplifier 1518 and signal processing unit 1519 may be similar toamplifier 1508 and signal processing unit 1509. As illustrated, an inputto amplifier 1518 is connected to shielding 1512 and another input isconnected to conductive element 1514. Coupling network 1511 includes aresistor R connected across the inputs to amplifier 1518. Electrode 1516is also connected to the input connected to shielding 1512.

In operation, electromagnetic signals 22 may be a time varying, verticalelectric field. The interaction of electromagnetic signals 22 withconductive element 1516 may cause and/or induce an electric response tobe conducted and/or transmitted to the input to amplifier 1518.Shielding 1512 may attenuate and/or prevent horizontal electromagneticsignals from reaching conductive element 1514. Accordingly, the signalsdetected by conductive element 1514 may represent a stable referencevoltage while the signals detected by conductive element 1516 mayrepresent may correspond to electromagnetic signals 22. Amplifier 1518may perform appropriate signal processing and output detectedelectromagnetic signals 22 to computing system 30. By using conductiveelement 1514 and shielding 1512, sensor 1562 may detect only thevertical components of electromagnetic signals 22. Accordingly, thedesign of sensor 1562 may be such that sensor 1562 does not respond tohorizontal components of electromagnetic signals 22 or otherelectromagnetic signals.

FIG. 15C illustrates current sensor 1564, which may be a particularembodiment of sensor 26 that includes shielding 1512, electrode 1516,coupling network 1513, resistor 1526, amplifier 1528, signalconditioning unit 1529, and battery 1530. Sensor 1564 may be capable ofdetecting electroseismic signals 22 may be capable of sensing signals 22as a current across a sense resistor 1526. Sensor 1560 may also output asignal representing detected electromagnetic signals 22. Sensor 1560 maybe installed and/or disposed in any appropriate housing, includingweather-resistant housing, movable vehicles, and/or permanentinstallations, as is discussed above with respect to sensor 26.

Shielding 1512 represents any suitable electromagnetic shielding, asdiscussed above with respect to FIG. 15A. Shielding 1512 may beconfigured to surround all or a portion of battery 1530. For example, asillustrated, shielding 1512 may comprise a structure that surrounds thetop and sides of battery 1530. Shielding 1512 may be electricallyconnected and/or coupled to an input to amplifier 1528. In particularembodiments, shielding 1512 may additionally or alternatively surroundall or a portion of coupling network 1513. As illustrated, shielding1512 surrounds sense resistor 1524 of coupling network 1513. As notedabove, it should be understood that in particular embodiments, shielding1512 may or may not be appropriate and/or necessary.

Coupling network 1513 may include any appropriate components operable tocouple battery 1530 to amplifier 1518. Coupling network 1513 may includesimilar components as discussed above with respect to FIGS. 15A and 15B.As illustrated, coupling network 1513 includes current sensor 1522 andsense resistor 1524. Current sensor 1522 represents any appropriatecurrent sensor operable to detect a current I generated by electrode1516. As illustrated, current sensor 1522 is a current transformer thatsenses current as a voltage drop across a sense resistor 1524. Thecurrent transformer may be a step-up transformer with, for example, upto 1000 times gain or more. Current sensor 1522 may represent anyappropriate current sensing technologies, including Hall effect sensors,a senseFET, or other appropriate current sensor.

Battery 1530 represents any appropriate voltage source operable to allowcurrent to flow from ground across sense resistor 1524. Battery 1530 mayhave a large self-capacitance. Charge may leak from ground and attemptto charge battery 1530. Battery 1530 may have a capacitance and/orresistance between the battery and ground, which may represent thecapacitance and/or resistance of air. Electrode 1516 may be connected toa terminal of resistor 1524. Resistor 1524 may be connected between theterminals of current sensor 1522. One terminal of resistor 1524 may beconnected to a terminal of battery 1530. Resistor 1526 may be connectedin parallel with battery 1530. The outputs of current sensor 1522 may beconnected to the inputs of amplifier 1528, which may provide an outputrepresenting electromagnetic signals 22. Amplifier 1528 and signalconditioning unit 1529 may be similar to amplifier 1508 and signalprocessing unit 1509. It should be noted that, in some embodiments,battery 1530 may additionally or alternatively comprise a capacitor. Itshould also be noted that in some embodiments, a current amplifier mayadditionally or alternatively perform the functions of current sensor1522, sense resister 1524, and amplifier 1528.

In operation, variations in ground potential caused by electromagneticsignals 22 and Earth's background electromagnetic field 14 may induce acurrent I across sense resistor 1524 that may be detected by currentsensor 1522. Amplifier 1528 and/or signal conditioning unit 1529 mayperform appropriate signal processing and output detectedelectromagnetic signals 22 to computing system 30.

It should be noted, however, that while FIGS. 15A, 15B, and 15Cillustrate particular embodiments of sensors 26, sensors 26 may includeany appropriate number and combination of components operable to detectportions of electromagnetic signals 22, such as various antennas orother sensing elements. Suitable antennas may include, but are notlimited to, a parallel-plate capacitor antenna comprising two or moreparallel conducting plates; a single-plate capacitor antenna comprisingone electrode electrically coupled to the earth; a monopole antennacomprising a conducting element, a dipole antenna comprising twoconducting elements; a multi-pole antenna comprising a plurality ofconducting elements; a directional antenna comprising conductingelements arranged to augment a signal amplitude in a particulardirection, and a coil antenna comprising one or more coils of wire,and/or any combination of suitable antennas. In some embodiments, sensor26 may represent a concentric electric dipole (CED). The CED may includetwo electrodes in a concentric configuration. For example, theelectrodes may be generally circular dipoles with an inner circularelectrode disposed concentrically within an outer circular electrode.The electrodes may generally be aligned in a plane that is parallel withthe plane of the surface of the earth. The CED may then preferentiallydetect the vertical portion of electromagnetic signals 22 that aresubstantially perpendicular to the plane of the CED. The verticalportion of electromagnetic signals 22 may create a detectable potentialdifference between the two electrodes.

In some embodiments, the electromagnetic sensor 26 may comprise a pairof electrodes in contact with the earth and disposed within the earth.For example, a first electrode may be disposed in a hole drilled intothe earth ranging from about 10 feet to about 15 feet. A secondelectrode may be disposed within about 1 foot to about 3 feet of thesurface of the earth, and the pair of electrodes may be electricallycoupled. In some embodiments, the pair of electrodes may be disposedwithin the earth at varying depths as needed to form an electricalcoupling with the earth. In some embodiments, the electrodes may takethe form of porous pot electrodes or other electrodes, such electrode1516. In some embodiments, the electrodes may comprise a conductiveelectrode in contact with the earth and electrically coupled to a porouspot electrode.

Monitoring Drilling Operations

FIGS. 3A and 3B are a flow chart of an example method according to thepresent disclosure for monitoring and controlling a drilling operationbased on electroseismic sensing while performing a drilling operation,which is designated generally by the numeral 300. The method of FIGS. 3Aand 3B may, for example, be used to determine the location of the drillbit or other portions of a drillstring in a wellbore at a time. Exampleimplementations may omit one or more of blocks 305-355, while in otherimplementations additional steps not specifically shown in FIGS. 3A and3B may be added. Still other implementations may perform one of more ofblocks 305-355 in an alternate order from the order shown in FIGS. 3Aand 3B.

A wellbore may be a horizontal deviated wellbore, such as the one shownin FIG. 2. In other implementations, the wellbore is vertical orsubstantially vertical. In still other implementations, the wellbore oneor more deviated segments include deviations between vertical andhorizontal.

In block 305, a first sensor array, including one or moreelectromagnetic sensors 26 and one more seismic sensors 28, are arrangedto monitor the drilling operation. In some implementations, the array ofsensors is located on or near the surface of the earth. In otherimplementations, one or more of the sensors are placed, at least inpart, just beneath the surface of the earth. In still otherimplementations, one or more of the sensors are located in the borehole.The placement of the first sensor array is discussed in greater detailbelow with respect to FIGS. 5-12. Example arrays of sensors include oneor more seismic sensors and one or more electromagnetic sensors. Incertain embodiments, the one or more seismic sensors and one or moreelectromagnetic sensors may be combined into a single unit. A examplesingle unit includes a geophone.

After the array of sensors are arranged according to the expected drillpath, and once drilling operations are under way, in block 310, thefirst sensor array receive seismic and electromagnetic signals generatedin the subterranean formation caused, at least in part, by the drillingoperation. In particular, the first sensor array receiveselectromagnetic signals that are caused by the electroseismic orseismoelectric conversion of the seismic signals generated by thedrilling operation. A drill bit generates seismic and electromagneticnoise as it penetrates the earth to form a wellbore. Both the seismicand electromagnetic signals caused by the drilling operation travel tothe first sensor array. The electromagnetic noise from the drill bitwill travel to the electromagnetic sensors 26 in the first sensor arrayat the speed of light in the formation. By contrast, the seismic noisefrom the drill bit will travel to the seismic sensors 28 at the speed ofsound in the formation. As will be discussed in greater detail below,the difference in detection time of the seismic and electromagneticsignals caused by the drilling operation is indicative of the locationof that drilling operation.

In some implementations, system 10 includes a second sensor arraylocated apart from the first sensor array. The second sensor array mayinclude one or more the electromagnetic sensors 26 and one or moreseismic sensors 28. In some implementations, the second sensor array islocated distant to the first sensor array, so that the effects of thedrilling operation will be minimized in the second sensor array. Thesecond sensor array may be placed apart from the first set of sensor ata distance substantially equal to or greater than a distancecorresponding to the depth of the drilling operation. The second sensorarray may be used by computing system 30 to, for example, removebackground noise from signals received at the first sensor array (block315). In other implementations, the signals from the second sensor arrayare used to determine one or more properties of the subsurface formationaway from the region where drilling is taking place. In certain exampleembodiments, this determination of properties of the subsurfaceformation is further based on signals from the first sensor array.

In some implementations, a seismic sensor 28, placed at substantialdistance from the drilling operations, generates a time-dependent signalreflective of the electric field generated in the Earth/air environment.In other implementations, an electromagnetic sensor 26 placed atelevation above the surveyed field detects the electric field in theair, distinct from that emanating from the earth. That time-dependentamplitude can be correlated by computing system 30 with the signalsdetected over a hydrocarbon field operation to model the subsurface.Alternatively, the time-dependence of the signal from the second set ofsensors may be used by computing system 30 to filter the data from thefirst sensor array to remove the background portion of the signal.

In still other implementations, the second set of sensors are placednear, but still removed from, the first sensor array and used to remove,for example, infrastructure noise. The second set of sensors may beplaced between the first set of sensors and the source of infrastructurenoise. Alternatively, the second set of sensors may be placed at alocation where the infrastructure noise generates any additional signalthat interferes with the first set of sensors. In one exampleimplementation, the second set of sensors is located near a road tomonitor and record the characteristic noises generated by the road.These characteristic noises of the infrastructure are then removed fromthe signals detected by the first sensor array by computing system 30.

Returning to FIG. 3, in block 320, computing system 30 may perform oneor more cross-correlations of signals from sensors. In certain exampleembodiments, the cross correlation is performed between sensors in thefirst sensor array. In certain example embodiments, the crosscorrelation is performed between sensors in the second sensor array. Incertain example embodiments, the cross correlation is performed betweensensors in the third sensor array, discussed below. In certain exampleembodiments, the cross correlation is performed between one sensor inthe first sensor array and a second sensor in the second sensor array.In certain example embodiments, the cross correlation is performedbetween one sensor in the first sensor array and a second sensor in thethird sensor array. In certain example embodiments, the crosscorrelation is performed between two sensors from two of the firstsensor array, the second sensor array, and the third sensor array

Certain implementations perform a cross correlation between time tracesignals from two seismic sensors 28 in the first sensor array to rejectnoise. This cross-correlation may be used to determine the noise thatmay be dominated by surface waves. The result of the cross correlationis a measure of surface noise that is of minimal value to determine thebehavior of the subsurface formation. In some implementations, theresult of this cross-correlation may be removed from the signal from thefirst sensor array by the computing system 30.

The computing system 30 may further compute one or more crosscorrelations of signals from the electromagnetic sensors to enhancethose signals. In certain implementations, the difference in electricalpotential measured between two electromagnetic sensors 26 isproportional to the horizontal electric field, while the potentialmeasured at a single electromagnetic sensor 26 is proportional to thevertical electric field. The horizontal electric field containsinformation that is characteristic of the source of electromagneticenergy, while the vertical electric field contains information that ischaracteristic of the signal from the returned electroseismicconversion. Then, if the difference in potential between twoelectromagnetic sensors 26 is cross-correlated with the sum of thesignals from two electromagnetic sensors 26, then the result will be thecrosscorrelation between the source signal and the returnedelectroseismic conversion. This crosscorrelation may suppress both thecommon noise between electromagnetic sensors 26 and the noise unique toone electromagnetic sensor 26. In some implementations, the resultantcrosscorrelation provides the travel time to the target.

Where implementations of system 10 include the first sensor array, theseismic or electromagnetic events that move progressively across thearray can be suppressed. These systematic events might be surfaceseismic waves propagating from great distance or localized sources frominfrastructure. Once those systematic surface noises are removed, thenoise on all sensors will be more symmetrical in the horizontal plane.The sum and difference of neighboring sensors in the first sensor arraywill then yield an even larger signal-to-noise ratio that is lessdirectionally dependent.

Returning to FIG. 3, in block 325, computing system 30 performs anauto-correlation of one or more signals from sensors in one or more ofthe first sensor array, the second sensor array, and the third sensorarray. In some implementations, this auto correlation may determine atime to synchronize the seismic sensors 28 in the first sensor array.The drill bit generates seismic noise as it cuts into the subsurfaceformation. That seismic noise also generates an electromagnetic responsethrough several mechanisms, including by electrokinetic coupling. Thegenerated electromagnetic wave travels to the surface at the speed oflight in the formation, which is much faster than the speed of theseismic wave generated by the drill bit. The computing system 30performs an autocorrelation of the time trace of the signal received atan electromagnetic sensor 26 in the first sensor array to determine atime lag between the arrival of the electromagnetic signal and thesubsequent arrival of the seismic response associated with the samedrilling event below the surface. The autocorrelation may be performedon any sensor sensitive to the source and the return signal. Forexample, a geophone detects both electromagnetic and seismic energy. Anautocorrelation on a geophone may detect the initial source EM signaland the subsequent return of the seismic wave. Example embodimentsinclude one or more capacitive sensors that, in turn, include anaccelerometer. A capacitive sensor that also contains an accelerometercan detect the source and the return signals on both the capacitivesensor and the accelerometer.

When the computing system 30 then applies the autocorrelation operationto each time trace generated by each seismic sensor 28 orelectromagnetic sensor 26 in the first sensor array, the resultant timetraces have the properties of conventional active-source seismicsignals. That is, the source electromagnetic signal in electroseismicsurveying sets the time for the start of a signal recording in the samemanner as a seismic source is synchronized with an array of geophones inconventional seismic surveying to set the start of the source signal.The autocorrelation of the electroseismic signal then has all theproperties of a single seismic time trace. Example embodiments featurethe simultaneous processing of the electromagnetic and seismic portionof the electroseismic and/or seismoelectric signals. The measuredelectric field is used as the reference for the subsequent arrival of aseismic wave.

The resulting pseudo-seismic time traces generated, at eachelectroseismic detector can be processed by all the methods known tothose skilled in the art of seismic processing. Such signal processingmight include one or more of velocity filtering, dip filtering,common-source-point stacking, static corrections, and migration todetermine the true location of a source.

Returning to FIG. 3, in block 330, in certain implementations thecomputing system 30 applies one or more filters to the signals from oneor more of the first sensor array, the second sensor array, and thethird sensors array. In certain example embodiments, the computingsystem 30 applies one or more velocity filters. In certain exampleembodiments, the computing system 30 applies one or more spatialfilters.

Certain example velocity filters are performed in the f-k domain. Thef-k domain is a plot of the frequency (f) versus wavenumber (k). Thewavenumber is the reciprocal of the spatial wavelength. The frequency isreciprocal of the arrival time at a seismic sensor 28 or electromagneticsensor 26. So a plot in f-k domain displays constant velocities asstraight lines.

Example spatial filter make use of the symmetry around the center lineof the lines of constant electromagnetic field, for example as shown inFIGS. 5-12. Example spatial filters make use of the vertical orhorizontal components of the electric field and seismic polarizationthat reverse sign on opposite sides of the center line. In certainexample embodiments, the vertical or horizontal components of theelectric field and seismic polarization are to stack data from multipleelectromagnetic sensor 26 or multiple seismic sensors 28. The differencein arrival time of the seismic signal as a function of distance from thecenter line can be used to determine downhole properties, such as thelocation of the drill bit or the location of the borehole. Thedifference in arrival time of the seismic signal as a function ofdistance from the center line, along with the arrival time of a seismicsensor 28 that is located directly over the well may be used todetermine the a three-dimensional location of the drill bit or formationproperty.

Example spatial filter may be used in certain example embodiments todistinguish between fractures propagating in the horizontal or verticaldirections when those fractures create predominantly horizontal orvertical dipolar structures.

In certain example embodiments the electromagnetic signal, directlypropagating from the well source, arrives at electromagnetic sensors 26in the sensor arrays at the same time. The seismic signal form the wellsource is received by the seismic sensors 28 at times related to thedistance from the center line. Filtering can be performed based on thedifference in arrival time between the electromagnetic and seismicsignals. To the extent that there are paired electromagnetic sensors 26and seismic sensors 28 at the same location (for example, a geophone),then each set of paired detectors has its own time mark at zero time,based on the arrival time of the electromagnetic signal at theelectromagnetic sensors 26. Such a detector with paired electromagneticsensors 26 and seismic sensors 28 does not need to be synchronized withthe other detectors in order to detect the “moveout” progression inarrival times. In certain example embodiments, each pairedelectromagnetic sensors 26 and seismic sensors 28 is used to produce anautocorrelation. The coputing system 30 may determine a systematicmoveout, based, at least in part, on the autocorrelation lag shiftsbetween paired electromagnetic sensors 26 and seismic sensors 28. Incertain example embodiments, the peak in the autocorrelation correspondsto the arrival of the drill bit. This peak in the autocorrelation willarrive at longer lags for electromagnetic sensors 26 and seismic sensors28 placed further from the center line.

In certain example embodiments the computing system 30 stacks thedetector signals from electromagnetic sensors 26 and seismic sensors 28directly. In certain example embodiments, after such stacking, the onlyremaining signal is the arrival of the electromagnetic pulse from depth.In certain example embodiments, the amplitude of this pulse from depthis a measure of the electrical resistivity of the formation.

As discussed above, the signal generated by the drill bit has bothseismic and electromagnetic components. The electromagnetic componentstravel to the earth's surface at the speed of light. That speed is muchgreater than the speed of travel for a seismic wave. Depending on thedistance between the sensors in the first sensor array, theelectromagnetic modulation caused by the drill bit penetrating the earthwill appear at the electromagnetic sensors 26 in the first sensor arrayat approximately the same time. Adding the autocorrelated time tracesfrom the electromagnetic sensors 26 in the first sensor array emphasizesthe signals arriving simultaneously. The assumption that electromagneticevents arrive substantially simultaneously is equivalent to saying thatthe electromagnetic modulation has near infinite velocity, which is muchlarger than the velocity of seismic events. The sum of all recordedelectromagnetic signals then discriminates against seismic events thatdo not arrive simultaneously at all detectors.

In certain implementations, detection of seismic arrivals is furtherrefined by applying a filter to the observed seismic wave (block 330).In certain example embodiments, the filter is a velocity-specificfilter. For example, seismic waves traveling on the earth's surface haveparticularly slow velocities. A surface seismic wave manifests as asignal systematically crossing the array of detectors with relativelylarge time delays between the arrivals of surface waves at detectors.Two types of surface waves with different velocities are Rayleigh wavesand Love waves. Another type of near-surface wave is the so called Lambwaves. These waves travel in the subsurface, propagating parallel to thesurface, in a depth of about 30 meters. Such waves are compressionalwaves, p-waves, which are guided by large seismic impedance contrastsnear the Earth's surface. Such waves are often detected after anearthquake. These waves travel at a velocity faster than Rayleigh wavesbut slower than bulk seismic waves. Seismic waves traveling in the bulkof the earth, “body waves,” have higher velocities. Such waves willarrive at the seismic detectors 26 with systematic time shifts that aresmaller than the times shifts associated with surface waves and Lambwaves. Two types of body waves are the so called p-waves orcompressional waves and the so called s-waves or shear waves.

In certain implementations, seismic signals traveling at differentvelocities are separated by applying a “dip” filter. This filter isapplied in the frequency/wavenumber domain. The frequency/wavenumberdomain applies to the seismic sensors 26. The wavenumber is proportionalto the reciprocal of the spacing between detectors on the array. It isfurther known that surface and body seismic waves have certaincharacteristic velocities specific to each geological environment.Knowledge of these velocities is used to define velocities that willpreferentially distinguish between waves traveling on the surface fromthose traveling in the subsurface. For example, the velocity filters atinfinite velocity for the EM waves, at Rayleigh and Love velocities forsurface waves, at Lamb velocities for near-surface waves, and at p-waveand s-wave velocities for the body waves, and the filter therebyeffectively isolates the various types of waves.

Returning to FIG. 3, in block 335, in certain implementations thecomputing system 30 performs signal processing on signals from one ormore of the first sensor array, the second sensor array, and the thirdsensor array.

In one example embodiments, the computing system performscommon-source-point processing. “Common-source-point” refers to therelevant signals that originate at the drill bit and then travel toseismic sensors 28. Here, the geometry of the expected signal from thedrill bit is illustrated in FIGS. 5-8. Certain example signals alsoinclude spherically symmetric components. The signals detected by thefirst array can therefore be processed as vertical and horizontalcomponents of the seismic response to yield the depth and horizontallocation of the drill bit.

For example, FIG. 5 shows that the vertical component of the seismicresponse changes sign on opposite sides of the symmetry point, which, inthis case is the location of the drill bit. The separation between themaximum amplitudes on the two sides of the symmetry point is equal tothe depth of the drill bit. The point of zero crossing locates thehorizontal position of the drill bit.

Processing the autocorrelation time traces provides the neededinformation. After velocity filtering and retaining the verticaldisplacement signal, the seismic arrival times and amplitudes at eachdetector are determined. A plot of amplitude-versus-horizontal distancedetermines the point of zero amplitude in both the x and y horizontaldirections. In FIG. 5, Z is the vertical coordinate. The signals fromsensors that are symmetrically located on opposite sides of this zeroamplitude location are then summed to suppress noise. The result is acurve passing through zero at the origin and displaying a maximum at adistance equal to one half of the depth of the drill bit. Such curvesare shown at the top of FIG. 5.

This geometrical calculation can be repeated for the horizontalcomponent of the seismic wave. As shown in FIG. 8, the horizontalcomponent of the seismic wave passes through zero above the location ofthe drill bit and the distance between the maxima is approximately equalto the depth. In one implementation, the horizontal amplitude of theseismic response is measured directly using a three component seismicsensor 28. In another implementation the horizontal amplitude of theseismic response is measured by computing the difference between theamplitudes detected on neighboring electromagnetic sensors 26. Similarprocessing can be applied to spherically symmetric components asillustrated in FIGS. 7 and 8, as discussed below.

Other example embodiments perform one or more other types of signalprocessing at block 335. For example, signal processing of block 335 mayinclude seismic processing may include time series analysis by Fouriermethods that include one or more of auto and cross correlation,convolution and deconvolution, Wiener filtering, multi-spectralanalysis, and Hilbert transforms.

Other example signal processing method may be used in the time seriesanalysis are also applied to the data collected from one or more sensorsof one or more of the first sensor array, the second sensor array, andthe third sensor array. In certain example embodiments, one or more ofthe first sensor array, the second sensor array, and the third sensorarray include a two-dimensional array of geophones or accelerometers. Incertain example embodiments featuring such an arrangement of sensors,two-dimensional Fourier transforms and velocity filtering may beperformed.

In certain example embodiments, arrays of seismic sensors 26 areemployed to stack data in various ways to enhance particular signalproperties. In certain example embodiments, For example, processingmight be done to add signals from one or more of the first sensor array,the second sensor array, and the third sensors that come from aparticular location between source and receiver, such as midway betweensource and receivers. This may be referred to as common midpointstacking processing (CMP). In other example embodiments, signalprocessing includes one or more of stacking with regard to a commonsource (CSP), a common receiver location (CRP), with respect to a fixeddepth, or common depth point stacking (CDP).

Seismic signals arrive at the various seismic sensors 28 in one or moreof the first sensor array, the second sensor array, or the third sensorarray at different times. This difference in time between the arrival ofseismic signals may be referred to as “moveout.” Example signalprocessing methods such as f-k filtering and moveout time-shiftcorrections are used for this purpose. In certain example embodiments,certain seismic velocities are observed. These seismic velocities mayinclude one or more of the velocity between two adjacent subterraneanlayers, the interval velocity, the effective velocity for a wavetraveling outward to successive receivers, the moveout velocity, and thevelocity used after all velocity corrections are used, which may bereferred to as the stacking velocity. Additionally, surface seismicwaves include Rayleigh and Love waves to designate the wavepolarization. These waves travel slowly compared to bulk waves. Analysisof signals from one or more of the first sensor array, the second sensorarray, and the third sensor array may be used to separate the arrivaltimes for surface waves relative to bulk waves. In other exampleembodiments, sensor arrays that includes one or more accelerometers orgeophones that measure three orthogonal components of vibration, mayseparate surface waves from bulk shear and compressional waves based ona wave's polarization and velocity.

When multiple seismic waves arrive at multiple detectors, the non-linearnature of the propagation path might lead to an inaccurate location of astructure in the subsurface. In certain example embodiments, the signalprocessing (block 335) includes migration to attempt to correct for thisnon-linear propogation.

Example processing methods that may be performed at block 335 rely onthe wave nature of a seismic wave. Seismic waves are reflected,transmitted, and refracted by the well-known Snell's laws. Waveformprocessing differs from much of electroseismic processing in that thewavelengths in the electromagnetic portion of an electroseismic signalis much larger than any structures of interest. In this case it may beappropriate to think of the electroseismic propagation problem in alow-frequency limit where there are no well-defined interfacereflections.

Based on the signal received from one or more of the first sensor array,the second sensor array, and the third sensor array, and subsequentprocessing, as described above with respect to blocks 315-335, thecomputer system 30 determines one or more drillstring properties. Oneexample drillstring property is the location of the drill bit. Incertain example embodiments the location of the drill bit relative to alocation at the surface or to a location within the subsurfaceformation. Other example drillstring properties include flexing orcorkscrewing of drill pipe. In certain example embodiments, the computersystem 30 monitors the changes in drillstring properties over time.Changes in the drillstring properties over time may indicate whether ornot drillstring components are functioning properly, or not. Forexample, change in drillstring properties over time may indicate that aportion of the drillstring has failed.

Based on the signal received from one or more of the first sensor array,the second sensor array, and the third sensor array, and subsequentprocessing, as described above with respect to blocks 315-335, thecomputer system 30 determines one or more formation properties (block340). These formation properties may include one or more formationproperties above, below, or in front of the drill bit. One exampleformation property is the presence of fluids in the subsurfaceformation. Another example formation property is the presence of a faultin the subsurface formation. Another example formation property is alocation of a change in formation layers. Other example formationproperties include one or more of the hardness of the rock in thesubsurface formation and the permeability or porosity of the subsurfaceformation. Other example formation properties include the proe pressureof the formation.

In certain example embodiments, the computing system 30 determines thelocation of the drill bit in the formation based, at least in part, onthe one or both of the first, second, and third arrays of sensors andthe results of one or more of blocks 325-335. For example, duringdirectional drilling the commuting system 30 may initially determine thelocation of the drill bit based on one or more surveys performed beforethe drilling operation and based on signals from one or more surveysensors located along the drill path. In certain embodiments, thecomputing system 30 modifies the calculated drill bit location based, atleast in part, on the signals from one or both of the first and secondsensor array or the results of one or more of blocks 325-335.

In certain example embodiments, the computing system 30 images theformation above, below, or beside the drill bit based, at least in part,on the signals from one or more of the first sensor array and secondsensor array and the results of one or more of blocks 325-335.

In certain example embodiments, the computing system 30 identifies thelocations of fluids, such as hydrocarbons, Helium, carbon dioxide, orwater, in the formation based, at least in part, on the or more ofsignals from one or more of the first sensor array and second sensorarray and the results of one or more of blocks 325-335.

In certain example embodiments, the computing system 30 identifies thelocations of faults based, at least in part, on signals from one or moreof the first sensor array and second sensor array and the results of oneor more of blocks 325-335. The identification of the location of thesefaults may then be used to alter the drill path or otherwise alter thedrilling operation. In still other implementations, the computing system30 determines where to initiate a completion or well enhancementprocedure, such as a fracturing stage based, at least in part, on thesignals from one or both of the first sensor array and second sensorarray and the results of one or more of blocks 325-335.

In certain example embodiments, the computing system 30 monitors theprogress of an enhanced oil recovery operation based, at least in part,on the signals from one or both of the first sensor array and secondsensor array and the results of one or more of blocks 325-335.

In certain example embodiments, the computing system 30 monitors otherwells operations based, at least in part, on the signals from one orboth of the first sensor array and second sensor array and the resultsof one or more of blocks 325-335.

In certain example embodiments, the computing system 30 performs qualitycontrol by identify fractures or damage created by well operationsoperation based, at least in part, on the signals from one or both ofthe first sensor array and second sensor array and the results of one ormore of blocks 325-335.

In certain example embodiments, the determination of one or moreformation properties includes determining an image of the subsurfaceformation above, in front of, behind, or below the drill bit.

In still other example implementations, the computer system 30determines one or more properties of an adjacent or distant well in thesubsurface formation. This may include the path of the adjacent ordistant well in the subsurface formation. Other example embodiments maydetermine one or more properties of the subsurface formation around theadjacent or distant well, such as the presence, location, or amount ofone or more fluids, such as hydrocarbons, Helium, carbon dioxide, orwater, in the formation around the adjacent or distant well based, atleast in part, on the signals from one or more of the first sensor arrayand second sensor array and the results of one or more of blocks325-335.

In block 350, the computing system 30 may receive signals from a set ofone or more third sensors. In certain example embodiments the signalsfrom the third sensor array include electromagnetic signals, which maybe cause by the electroseismic or seismoelectric conversion of seismicsignals caused by the drilling operation. In some exampleimplementations, at block 350 the computing system 30 determines orupdates one or more of a drillstring property and a formation propertybased, at least in part, on the one or both of the first, second, andthird arrays of sensors and the results of one or more of blocks325-335. In one example embodiment, the computing system updates one ormore drillstring properties or formation properties based on adifference in time between electromagnetic signals received at one ormore of the first sensors array, the second sensor array, and the thirdsensor array.

In block 355, in certain example embodiments the computing system 30further tests the electrical conductivity during the drilling operation.In general, the electrical conductivity is another way to detect changesin fluid content and lithology. In certain implementations, theelectrical conductivity is correlated with a seismic response of onemore seismic sensors 28 in the first sensor array. In one exampleimplementation, in combination with an electroseismic survey duringdrilling, an electrical voltage is applied between the well and adistant electrode. In certain example implementations, one electricalcontact is made to one or more of the well head, the drillpipe, and thedrill bit. A second electrode may be located such that a current iscreated through a region of interest. The second electrode may belocated at a second position on the drilling equipment, and/or on aneighboring well casing or drilling equipment, and/or on infrastructuresuch as pipes and/or fences, and/or the second electrode may be anelectrode placed on the surface of the Earth. In certain exampleembodiments, the surface electrode is located between approximately 10feet and 20,000 feet from the well location. One or more electroseismicsensors 26 in the first sensor array are configured to detect anelectrical voltage contemporaneously with and/or at a time differentfrom the seismic data collection. Alternatively, the electromagneticsensor may be separate from the seismic sensor 26. When the drill bitencounters a region of high electrical conductivity, such as a waterpocket, the resistivity will decrease along with the electroseismicresponse in the electroseismic sensors 26 in the first sensor array. Thecombined effect favors the change in fluid conductivity.

In block 360, the computing system 30 alters the drilling operationbased on the determined drill bit location from the previous steps. Insome example implementations, the computing system 30 causes the changein weight-on-bit of the drilling. In other implementations, thecomputing system 30 actives a mud motor in the drillstring to alter thedirection of the drillstring. In still other implementations, thecomputing system 30 causes the drillstring to come off bottoms and tripto a location. In other example embodiment the computing system 30alters the rate of penetration of the drilling operation.

The placement of the first sensor array will be discussed with respectto FIGS. 5-12. FIG. 5 is a cross-section view in a plane containing awell 505 with both vertical and horizontal sections. The drill bit 510is in a horizontal portion of the wellbore 505 within reservoir 515. Asthe drill bit 510 drills in the subsurface formation, the acoustic noisefrom a drill bit 515 generates seismic and electromagnetic noise. In oneimplementation, the noise may take the form of dipolar radiation. Incertain implementations, these noise signals travel to the surface insuch a way that the symmetries of the noise signals, as received at thesurface, determine the depth of the drill bit 515. In FIG. 5, themaximum horizontal electric field and seismic amplitudes occur at 30degrees from the head of drill bit 515, as shown. In thisimplementation, the distance between maxima is equal to the depth. Thedifference between the signals recorded at +/−30 degrees is the sum ofthe maximum amplitudes and removes common signals from both detectors.The depth of the drill bit 515 can also be determined based on thetravel time of the seismic signal to the surface. The seismic traveltime increases with offset, i.e., with distance from the center lineover the drill bit. Although the wellbore 505 of FIG. 5 has a horizontalsection, other wellbores 505 will be vertical or substantially vertical.Still other wellbores 505 will include one or more deviated sectionswith deviations between vertical and horizontal.

FIG. 6 is a cross-section view in a plane containing a well 505 wherethe plane of view is perpendicular to the plane of FIG. 5. Note that theelectric field and seismic amplitudes exhibit an identical behavior asin FIG. 5. The first sensor array can therefore be arranged to capture athree-dimensional image of a point source of seismic or electromagneticnoise. The third dimension location of the drill bit 510 is determinedby symmetrically placed sensors in a plane perpendicular to thehorizontal well. These geometrical relationships apply to anyacoustic/seismic dipolar disturbance generated at a point in thesubsurface, for a “point” with a radius smaller than the first seismicFresnel zone of a seismic wave at the measured frequency and depth.

Example arrangements of sensors take many forms, including, for example,arrays on a rectangular grid and other arrangements that are commonlyused in seismic imaging. Additionally, an array may be formed by lineararrangements of sensors parallel and/or perpendicular and/or at an angleto the path of the drilling operation. Additionally, sensors may beplaced at random locations of known position.

FIG. 7 is a cross-section view in a plane containing a well 505 that hasbeen drilled into reservoir 515 and fractures 705 have been induced inthe reservoir 515. The top of FIG. 7 illustrates the electric fieldcaused by fluid flowing out of fracture 705 and through a productioncasing. In certain implementations, fluid flowing out of a fracture 705creates a streaming potential by electrokinetic coupling. In the case offracture generation, the seismic and electrical amplitudes are expectedto be substantially larger than amplitudes generated by the flow offluid in production. In the fracturing process, fluid flows into thereservoir 515 to form fracture 705. The fracture generation processresults in substantial energy being applied to the reservoir 515 tofracture the rock. This applied energy creates both a seismic andelectroseismic response that can be detected by the array of sensors.

The situations described in FIGS. 5-7 assume vertical mechanical orfluid motion, which is equivalent to a vertical dipole source. The drillbit 510, however, also has horizontal motion similar to the verticalmotion. Likewise, the fracture 705 has horizontal amplitudes that may aslarge as, or larger than, the vertical displacement of the fracture 705.This might occur because the expansion of a fracture in the direction ofminimum stress generates more rock movement, a larger volume of rockmovement, than does the vertical propagation of a fracture 705. This isalso the case in fluid flow in fractures that connect subterraneanstructures with extensive horizontal branches.

A mechanical disturbance with horizontal and vertical components can besplit into orthogonal horizontal and vertical polarizations, if it isnot spherically symmetrical. Equivalently, in certain exampleimplementations the signal can be separated into horizontal and verticaldipoles.

As shown in FIG. 8, for horizontal polarizations, the signal geometry isthe same as for the vertical polarization; however, the signal geometricsignatures are reversed. For horizontal source polarizations, thevertical components of the electric field and seismic amplitude on thesurface flip signs on opposite sides of the symmetry line or point. Thehorizontal components of the electromagnetic and seismic signals at thesurface are continuous with a maximum at the symmetry point. The time ofarrival of a seismic response, combined with the signal geometry, definethe location of the drill bit 510 for either or both polarizations.

In certain implementations, changes of sign on opposite sides of thesymmetry point can be measured, as shown in FIGS. 6-8. The geometricalproperties of the signal can therefore be determined by deploying anarray of sensors on the surface. These sensors may include one or moreelectric-field sensors, magnetic-field sensors, one-component seismicsensors, two-component seismic sensors, or three component seismicsensors.

FIGS. 9 and 10 show example expected vertical electric field amplitudesgenerated by a horizontal electric dipole at depth. The horizontaldistance is expressed in units of the depth to the target. FIG. 10 showsthe offset distance over which the electric field is at least one halfof the maximum electric field. The electric field changes sign atnegative offsets, i.e., on the other side of the point or line of originof the dipole.

Based on the expected results shown in FIG. 10, for a 10,000 foot deeptarget, example sensors may be placed at 2,000 feet (depth to target) to19,000 feet (depth to target) from a vertical plane passing thought thelater. In this range of distances, the signal amplitude at the sensorswill be at least half the peak amplitude. The peak amplitude of thisexample is at 7,000 feet.

In certain example implementations, for the horizontal electric dipoleat depth, the peak amplitude of the vertical electric field occurs at adistance of 0.7 times the depth of the target. This corresponds to anangle of 44 degrees between the vertical direction and the direction ofmaximum amplitude. For a vertical dipole at depth, the horizontalelectric field on the surface is similar in appearance. The position ofmaximum amplitude for the vertical dipole occurs at a distance of 0.5times the depth to target, which corresponds to an angle of 30 degrees.The case of the vertical dipole is illustrated in FIGS. 1 and 2.

FIGS. 11 and 12 are similar to FIGS. 9 and 10, but are for an examplespherically symmetric source of electromagnetic energy at depth. Thehorizontal electric field reverses sign on the opposite side of thesymmetry point. The figures show that the offset dependence has adifferent shape for the dipole and spherical cases. The difference inshape is important. If the source is a vertical or horizontal dipole,the electric field is large at smaller offsets. In this case, a smallerfootprint of the first sensor array on the surface will enable signaldetection if the source has a dipole configuration.

Monitoring and Controlling Drilling Operations Using Percussive Drilling

In certain example embodiments, the drill bit is controlled to produce adetectable pattern in the resultant seismic energy received at seismicsensors 28. For example, the drilling operation (block 310) may be apercussive drilling operation in which a controlled vibration isimparted to the drillstring during the drilling operation. An example ofsuch a percussive drilling operation is discussed in U.S. Pat. No.8,517,093, entitled “System and Method for Drilling HammerCommunication, Formation Evaluation and Drilling Optimization,” by ToddW. Benson, the contents of which are incorporated by reference herein.In certain example embodiment of the controlled-vibration percussivedrilling, the vibration is controlled to occur at a known sequence.Example sequences are in the family of sequences known as Golaycomplementary sequences. Other example sequences are Barker sequences.In other example embodiments, the percussive drill bit is programmed toprovide an impulse to the drill bit in a controlled sequence. In certainexample embodiments, the drill bit is programmed to provide aquasi-periodic or pseudo-random sequence of pulses. In certain exampleembodiments, the drill bit is programmed to provide a sequence withvarying amplitude. In certain example embodiments, the drill bit isprogrammed to provide a sequence with varying frequency.

The frequency of the vibrations induced in the drillstring may becontrolled to, for example, enhance the signal-to-noise ratio of aresulting signal at a seismic sensor 28. In certain implementations ofthe controlled-vibration percussive drilling, the amplitudes of thevibration induced in the drillstring are also controlled. This may beused for example, to increase the signal-to-noise ratio in signalsreceived at seismic sensors 28. Vibrations resulting from thecontrolled-vibration percussive drilling are received at one or more ofthe seismic sensors 28. The computer system 30 performs correlation ofthe series of signals that were imparted to the drillstring during thecontrolled-vibration percussive drilling with the signals received atthe seismic sensors 28. In some example implementations, the result ofthis correlation is used to determine the location of the drill bit inthe formation. The result of this correlation may further be used toimage the formation above, below, or beside the drill bit. The result ofthis correlation may further be used to identify the locations offluids, such as hydrocarbons, Helium, carbon dioxide, or water, in theformation. The result of this correlation may further be used to monitorthe progress of an enhanced oil recovery operation. The result of thiscorrelation may further be used to monitor other wells.

The result of this correlation may further be used to perform qualitycontrol by identify fractures or damage created by well operations.

The result of this correlation may further be used for geosteering ofthe drill bit to a location in the formation that is likely to producegreater amounts of fluid. In other example embodiments, thedetermination of the drill bit location is used for geosteering aroundfaults.

FIG. 16 is a flow chart of an example method according to the presentdisclosure for surveying a formation. In block 1605, the system performsa survey of subsurface earth formation 16 before the drilling operation.In certain example embodiments, the survey of block 1605 is apassive-source electromagnetic survey, as described above with respectto FIGS. 1 and 2. Thereafter, the system monitors the drilling operation(block 300), as discussed with respect to FIGS. 3A and 3B. In certainexample embodiments, the system further performs a survey after thedrilling operation (block 1610). In some embodiments, blocks 1605, 300,and 1610 are performed using a common set of electroseismic sensors 26and seismic sensors 28, such as one or more of the first sensor array,the second sensor array, and third sensor array, as discussed above.

In certain example embodiments, the survey of block 1610 is apassive-source electromagnetic survey, as described above with respectto FIGS. 1 and 2. In certain example embodiments, further drillingoperations may be performed after block 1610. For example, correctionsto the drill path may be made based on the results of the survey afterthe drilling operation.

Monitoring and Controlling Fracturing Operations

FIGS. 13A and 13B are flow charts of an example method according to thepresent disclosure for monitoring and controlling a fracturing operationbased on electroseismic sensing while performing the fracturingoperation. The method is referred to generally by the numeral 1300. Themethod of FIGS. 13A and 13B may, for example, be used to determine theorientation and progression of fractures in a subterranean formation ata time. Example implementations may omit one or more of the blocks showin 13A and 13B, while other implementations additional steps not shownin FIGS. 13A and 13B. Still other implementations may perform one ofmore of block in FIGS. 13A and 13B in an alternate order from the ordershown in FIGS. 13A and 13B.

The processing described in FIGS. 13A and 13B is similar to theprocessing described above with respect to FIGS. 3A and 3B, with commonelements sharing common numbering with blocks from FIGS. 3A and 3B. Thedifferences between the two will be discussed below.

In block 1305, a first sensor array, including one or moreelectromagnetic sensors 26 and one more seismic sensors 28 are arrangedto monitor the fracturing operation. In certain example implementations,the seismic sensors 28 and electromagnetic sensors 26 may be part of asingle unit, such as a geophone. In general, fractures will be initiatedover a finite length of cased well. In some implementations the array ofsensors are located on or near the surface of the earth. In otherimplementations, one or more of the sensors are placed, at least inpart, just beneath the surface of the earth. In still otherimplementations, one or more electromagnetic sensors 26 or one moreseismic sensors 28 are located at a location in or along the borehole.Example systems include one or more electromagnetic sensors 26 withcontact measurement to measure the electrical potential on the inside ofa borehole casing to thereby detect an electroseismic signal. Theplacement of the first sensor array is discussed in greater detail withrespect to FIGS. 5-12 and, in particular, FIG. 7.

In block 1310, the computer system 30 receives one or more seismic andelectromagnetic signals generated in the subterranean formation duringthe fracturing operation. The fracturing operation generates seismic andelectromagnetic noise as it penetrates the earth to form a wellbore.Both the seismic and electromagnetic signals caused by the fracturingoperation travel to the first sensor array. The electromagnetic noisefrom the drill bit will travel to the electromagnetic sensors 26 in thefirst sensor array at the speed of light in the formation. Theelectrometric signal may be cause by the electroseismic orseismoelectric conversion of the seismic signal from the fracturingoperation. By contrast, the seismic noise from the drill bit will travelto the seismic sensors 28 at the speed of sound in the formation. Insome implementations, processing signals generated by fracturing issimilar to processing signals caused by the drilling of the borehole. Insome implementations, fractures may occur along a finite length of wellcasing and the fractures may spread out a substantial distance involume. The relevant length scale for these issues is the radius of thefirst seismic Fresnel zone. In some implementations this dimension maybe 100 feet or more. Fractures are normally formed over a length ofcasing that is sealed by packers or plugs. If the casing interval ismuch longer than the radius of the first Fresnel zone, then the sourceis interpreted as a finite body, rather than a point source.

In some implementations, for a finite, linear body, the geometry of thesignals illustrated in FIGS. 5-12 will no longer be symmetrical in thex-y plane. Instead, in certain implementations, the use of thecommon-source-point processing will have different dimensions in the xand y directions. In other implementations, however, the fracturingoperation may be modeled as a point source, as with the drillingoperation.

In block 1315, the computer system 30 determines one or more of afracture property and a formation property based, at least in part, onsignals from one or more of the first array or sensors, the secondsensor array, the third sensor array, and one or more processing steps315-335. In one example embodiments, the computer system 30 determinesone or more properties of the formation, such as the presence andlocation of fluids, including hydrocarbons or Helium in the subsurfaceformation. In general, the computer system 30 may determine theformation properties as described above with respect to block 345. If adrillstring is present in the formation during fracturing, the computersystem 30 may determine drillstring properties, as described above withrespect to block 340.

In certain example implementations, the computer system 30 determinesone or more properties of the fracture being initiated or otherfractures. In one example embodiment, the computer system 30 determinesthe orientation of the fracture. In one example embodiment, the computersystem 30 determines the extent of the fracture. In one exampleembodiment, the computer system 30 determines the density of thefracture. In one example embodiment, the computer system 30 determinesthe porosity or permeability of the fracture. In one example embodiment,the computer system 30 determines the connectivity of the fracture. Inone example embodiment, the computer system 30 determines the size orshape of the fracture.

In block 1320, the computer system 30 tests electrical conductivityduring the fracturing operation. In general, fractures are expected tobe complex structures with high surface area and high electricalconductivity. Because the fractures increase the area of the exposedfluid in the formation, the measured conductivity increases as thefracture propagates in the subsurface formation. On the other hand, insome implementations, poor fractures are detectable based on smallchanges in resistivity while the fracture is initiated or propagated. Insome implementations, this conductivity testing will qualitativelyassess fracturing as a function of the position in the well. Thismeasurement may also reveal low hydrocarbon saturations when theresistivities are low and the measured electroseismic amplitude issmall.

In block 1325, the computer system 30 alters the fracturing operationbased on one or more of the determine fracture property and thedetermine formation property. In one example embodiment, that computersystem 30 alters the fracturing operation based, at least in part, onthe determined fracture orientation and propagation. In some exampleimplementations, the location or orientation of the fracture is altered.In other implementations, the fracturing pressure is altered. In someimplementations, the fracturing fluid or the proppant is altered.

In certain implementations, passive electroseismic sensing is used toimage a well environment before and/or after fracturing. The resultingimaging may be used to determine one or more issues involving thequality of the fracturing. For example, the computer system 30 maydetermine the size of the fractures, the location of one or morebypassed areas, fracturing in unintended formations, or water invasion.

FIG. 17 is a flow chart of an example method according to the presentdisclosure for surveying a formation. In block 1705, the system performsa survey of subsurface earth formation 16 before the fracturingoperation. In certain example embodiments, the survey of block 1705 is apassive-source electromagnetic survey, as described above with respectto FIGS. 1 and 2. Thereafter, the system monitors the fracturingoperation (block 1300), as discussed with respect to FIGS. 13A and 13B.In certain example embodiments, the system further performs a surveyafter the fracturing operation (block 1710). In some embodiments, blocks1705, 1300, and 1710 are performed using a common set of electroseismicsensors 26 and seismic sensors 28. In certain example embodiments, thesurvey of block 1710 is a passive-source electromagnetic survey, asdescribed above with respect to FIGS. 1 and 2. In certain exampleembodiments, further fracturing operations may be performed after block1710. For example, a new fracture may be initiated at a location basedon the results of one or both of blocks 1300 or 1710. In still otherembodiments, one or more subsequent surveys are performed to determinethe performance of the fracturing operation over time. For example, thesubsequent surveys may determine if the induced fractures are closing oraltering shape over time. In certain example implementations, the timebetween the time between surveys may be measured in seconds (forexample, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54,55, 56, 57, 58, or 59 seconds). In other example implementations, thetime between surveys may be measured in minutes (for example, 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58,or 59 minutes). In other example implementations, the time betweensurveys may be measured in hours (for example, 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, or 23 hours). Inother example implementations, the time between surveys may be measuredin days (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, or 31 days).In other example implementations, the time between surveys may bemeasured in months (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or12 months). In still other example implementations, the time betweensurveys may be measured in years. For example, the computer system 30may receive regular measurements from the set of electroseismic sensors26 and seismic sensors 28.

Monitoring and Controlling Production Operations

FIGS. 14A and 14B are flow charts of an example method according to thepresent disclosure for monitoring and controlling the production offluids, such as hydrocarbons, Helium, carbon dioxide, or water, from asubsurface formation. The method of FIGS. 14A and 14B may, for example,be used to determine the magnitude of production from a subsurfaceformation and the locations of depletion of that formation. Exampleimplementations may omit one or more of the blocks shown in in 14A and14B, while other implementations additional steps not shown in FIGS. 14Aand 14B. Still other implementations may perform one of more of block inFIGS. 14A and 14B in an alternate order from the order shown in FIGS.14A and 14B.

The processing described in FIGS. 14A and 14B is similar to theprocessing described above with respect to FIGS. 3A and 3B, with commonelements sharing common numbering with blocks from FIGS. 3A and 3B. Thedifferences between the two will be discussed below.

In block 1405, a first sensor array, including one or moreelectromagnetic sensors 26 and one more seismic sensors 28 are arrangedto monitor the production operation. In some implementations the arrayof sensors are located on or near the surface of the earth. In otherimplementations, one or more of the sensors are placed, at least inpart, just beneath the surface of the earth. The placement of the firstsensor array is discussed in greater detail with respect to FIGS. 5-12and, in particular, FIG. 7.

In block 1410, the computer system 30 receives one or more seismic andelectromagnetic signals generated in the subterranean formation duringthe production operation. Processing the data from fluid productiondiffers from processing drill-bit and fracturing data. First, the flowof fluids will be distributed over a length of pipe that will not, ingeneral, be a point source. In some example implementations, the flow offluid out of fractures might create a large electric field without alarge seismic response. In some example implementations, the pressureand electric field fluctuations are distributed within the productionfluid and over the length of the production casing. A single event, froma single fracture, will be masked by many events, from many producingfractures.

In the case of long horizontal wells, the first sensor array may resolveflow through sections of pipe. In certain implementations, these flowrate changes are shown as changes in electric field with distance.

In certain implementations, the computer system 30 receives one or moreseismic and electromagnetic signals generated in the subterraneanformation during a pressure test. Due to the nature of such a test, thissituation may provide usable electroseismic data to the first sensorarray.

In block 1415, the computer system 30 tests electrical conductivityduring the production operation. During production the electricalconductivity will systematically change as the reservoir is depleted. Incertain implementations, one or more electromagnetic sensors are used tomeasure the electroseismic response and the electrical conductivityalong the production pipe can reveal zones of poor productivity.

In block 1420, the computer system 30 alters production operations basedon the measurements from the first sensor array. In someimplementations, this includes altering a pumping pressure or shuttingin the well. In some implementations, this includes altering a rate ofproducrtion of the well. In still other example embodiments the systemmay change from convention production to an enhanced oil recoverymethod.

Subterranean Reservoir Zone Evaluation

FIG. 18 is a flow chart of an example method according to the presentdisclosure for evaluating one or more reservoir properties. In certainexample embodiments the system may be used to monitor the production offluids from a subterrenean formation. In still other embodiments, thesystem may be used for locating areas of fluid production from asubterranean reservoir. The methods of FIG. 18 may, for example, be usedto determine one or more properties of a producing reservoir, includingdetermining the delineation (e.g., edges) of one or more reservoirs. Orthe method of FIG. 5 may be used to identify a new reservoir forproduction. Example implementation may omit one or more of blocks1805-1835, while other implementations may include additional steps notspecifically shown in FIG. 18. Still other implementations may performone of more of blocks 1805-1835 in an alternate order from the ordershown in FIG. 18.

In block 1805, a first array of sensors, are arranged to monitorproduction of fluids, such as hydrocarbons, from a reservoir in thesubterranean formation. In certain example embodiments the first arrayof sensors includes one or more electromagnetic sensors 26. In stillother embodiments, the first array of sensors may include one or moreseismic sensors 28. In some implementations, the array of sensors islocated on or near the surface of the earth. In other implementations,one or more of the sensors are placed, at least in part, just beneaththe surface of the earth. Example sensors include a ground portion thatis placed in the Earth or that is attached to a grounded element.Certain of the electromagnetic sensors 26 may be permanently installedat desired locations. In still other implementations, one or more of thesensors are located in the borehole.

In block 1810, the electromagnetic sensors 26 receive a set ofelectromagnetic signals that are generated by the electroseismic orseismoelectric conversion of seismic signals in the subterraneanformation. The seismic signals in the subterranean formation includethose generated by the movement of fluids from a reservoir in thesubterranean formation during production. The seismic signals in thesubterranean that are generated by the movement of fluids from areservoir in the subterranean formation during production may bereferred to as type of passive source seismic signals.

In certain example implementations, in block 1815, the first array ofsensors is repositioned to new locations. For example, the one or moreof the sensors in the array of sensors may be moved as production of thereservoir progresses to more accurately monitor the ongoing production.In other implementations, however, the first array of sensors is notmoved between one or more surveys.

Returning to FIG. 18, in block 1820, at a second time, theelectromagnetic sensors 26 receive a set of electromagnetic signals thatare generated by the electroseismic or seismoelectric conversion ofpassive source seismic signals in the subterranean formation. Certainimplementations receive subsequent sets of electromagnetic signals thatare generated by the electroseismic or seismoelectric conversion ofpassive source seismic signals in the subterranean formation over time.In some implementations, the location of the sensors is changed betweensubsequent surveys, while in other implementations the sensors are leftat the same location. In certain example implementations, the timebetween the time between surveys may be measured in seconds (forexample, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54,55, 56, 57, 58, or 59 seconds). In other example implementations, thetime between surveys may be measured in minutes (for example, 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58,or 59 minutes). In other example implementations, the time betweensurveys may be measured in hours (for example, 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, or 23 hours). Inother example implementations, the time between surveys may be measuredin days (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, or 31 days).In other example implementations, the time between surveys may bemeasured in months (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or12 months). In still other example implementations, the time betweensurveys may be measured in years. A person of ordinary skill in the artwould further recognize that the time between surveys may be measured ina combination of the above units. In still other example embodiments,the time between surveys may be based on a user requested time.

In block 1825, the computing system 30 uses one or more the receivedsets of electromagnetic signals to determine one or more reservoirproperties. In certain example embodiments, the reservoir propertiesinclude the presence, location, and amount of fluids in the reservoir.Example fluids may include one or more of hydrocarbons, water, helium,or carbon dioxide. Other example reservoir properties include one ormore of pore pressure or permeability. In certain implementations, thecomputer system 30 relies on one or more available seismic surveys andproduction surveys to determine one or more reservoir properties.

In one example embodiment, the computing system 30 determines thelocations of one or more zones of production. In other implementations,the computer system 30 further relies on one or more available seismicsurveys and production surveys to determine zones of production. In someexample implementations, computing system 30 compares the survey resultsfor a zone not currently under production with a zone currently underproduction to determine an expected production capability of the zonenot currently under production.

In block 1830, the computing system 30 uses one or more the receivedsets of electromagnetic signals to determine one or more delineations(e.g., edges) of the formation.

Based on the determined zone of production (block 1825) and/or thedetermined locations of reservoir delineations (block 1830), the systemmay alter production operations (block 1835). For example, the systemmay determine that one or more infill or step-out wells should bedrilled to enhance production. Other enhanced oil recovery (EOR)operations may include chemical flooding, miscible displacement, andthermal recovery. In certain example implementations, the operator willperform hydraulic fracturing and the administration of a proppant to thesubterranean formation.

Long-Term Monitoring of Fluid Production from a Reservoir

FIGS. 19A and 19B are a flow chart of an example method according to thepresent disclosure for monitoring the fluid production from asubterranean reservoir. The methods of FIGS. 19A and 19B may, forexample, be used to evaluate the sweep efficiency of productionoperations. Or the methods of FIGS. 19A and 19B may be used to identifycandidates for EOR operations. Example implementation may omit one ormore of blocks 1905-645, while other implementations may includeadditional steps not specifically shown in FIGS. 19A and 19B. Stillother implementations may perform one of more of blocks 1905-1945 in analternate order from the order shown in FIGS. 19A and 19B.

In block 1905, a first array one or more sensors, including one or moreelectromagnetic sensors 26, are arranged to monitor production offluids, such as one or more of hydrocarbons, water, helium, or carbondioxide from a reservoir in the subterranean formation. In someimplementations, the array of sensors is located on or near the surfaceof the earth. In other implementations, one or more of the sensors areplaced, at least in part, just beneath the surface of the earth. Examplesensors include a ground portion that is placed in the Earth or that isattached to a grounded element. The grounded connectors can be installedconveniently by permanently burying electrodes at surveyed locations.Continuous or periodic measurements can be made by connecting therequired electronics to these buried electrodes. In otherimplementations, the grounded connections can also be made to certaininfrastructure, such as pipes, fences, and wells. In implementations tomonitor the reservoir over a long period of time, it is oftenadvantageous to permanently install the electromagnetic sensors 26 atdesired locations. In still other implementations, one or more of thesensors are located in the borehole.

In block 1910, the array of sensors receives a set of electromagneticsignals that are generated by the electroseismic or seismoelectricconversion of passive source seismic signals in the subterraneanformation. The array of sensors continues to receive second andsubsequent signals over time. In certain example implementations, thetime between the time between surveys may be measured in seconds (forexample, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54,55, 56, 57, 58, or 59 seconds). In other example implementations, thetime between surveys may be measured in minutes (for example, 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58,or 59 minutes). In other example implementations, the time betweensurveys may be measured in hours (for example, 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, or 23 hours). Inother example implementations, the time between surveys may be measuredin days (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, or 31 days).In other example implementations, the time between surveys may bemeasured in months (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or12 months). In still other example implementations, the time betweensurveys may be measured in years. For example, the computer system 30may receive regular measurements from the sensor array. Based on theresults of these measurements, the computer system 30 monitors theproduction and movement of fluids from the reservoir (block 1920).

In certain implementations, the formation will undergo EOR to enhanceproduction. This may include flooding the formation with water, steam,or other fluids. In such a situation, the sensor array may be used totrack the flood in the formation by comparing signals from the sensorarray during the flooding operation with prior sensor measurements. Achange in electroseismic amplitude of the signal from the sensor arraywill correspond to an interface between a hydrocarbon, such as oil, anda flooding agent, such as water, steam, or chemical flood. This is dueto the difference in electroseismic response between the hydrocarbon orother fluid, on the first hand, and the flooding agent, on the secondhand.

In certain implementations, the method further includes determining anamount of fluid that can be produced from the reservoir based, at leastin part, on the signals from the sensor array (block 1930). Theelectroseismic amplitudes will vary based on the fluid content of thesubsurface. In certain implementations, a rock saturated withhydrocarbon has different electroseismic amplitudes than a rocksaturated with EOR fluid. These differing amplitudes enable the computersystem 30 to track an interface between two fluids in the formation. Incertain implementations, the interface progression is correlated withthe amount of oil produced. In such an implantation, the interfacemovement is a measure of the sweep efficiency and of the volume of thereservoir interval.

In block 1935, the method includes detecting bypassed fluids. Asdiscussed above in block 1930, the computer system 30 tracks theprogression of the interface between fluids in the formation. In certainexample situations, the interface has a complex shape. For example, whenthe interface between oil and an EOR fluid progresses in a complexgeometrical shape, this may indicate the existence of bypassed fluids orcomplexity in the reservoir. In certain implementations, the bypassedfluids yield large electroseismic amplitudes. In certainimplementations, complexity in the reservoir rock that does not containoil should yield small electroseismic amplitudes.

The method further includes detecting unintended fluid migration inblock 1940. In certain implementations, this process is used for qualitycontrol. The electroseismic amplitude at the reservoir depth, neardrilling operations, and/or producing facilities, may indicateunintended hydrocarbon migration away from a reservoir, productionoperations, or drilling facilities. In certain implementations, thesensor measurements are useful to track the migration of pollutants nearthe Earth's surface.

Based, for example, on the detection of bypassed fluid (block 1930), thesystem may alter production operations (block 1945). For example, when aproduction front proceeds in an irregular fashion, it may revealsubstantial volumes of unswept fluid. In certain implementations, theunwept fluids manifest as areas of high electroseismic amplitude that donot change over time. Comparing the electroseismic properties of theseunswept regions with the electroseismic properties known regions of goodproductivity, the computer system 30 may determine locations where thereis high probability for producing additional fluids. The computer system30 may then determine that one or more infill or step-out wells shouldbe drilled to enhance production. Other enhanced oil recovery (EOR)operations may include chemical flooding, miscible displacement, andthermal recovery. In certain example implementations, the operator willperform hydraulic fracturing and the administration of a proppant to thesubterranean formation.

Detection of Reservoir Connectivity

FIG. 20 is a flow chart of an example method according to the presentdisclosure for determining the connectivity of reservoir segments basedon passive electroseismic surveying. The methods of FIG. 20 may, forexample, be used to determine the connectedness of a reservoir interval.Example implementation may omit one or more of blocks 2005-2020, whileother implementations may include additional steps not specificallyshown in FIG. 20. Still other implementations may perform one of more ofblocks 2005-2020 in an alternate order from the order shown in FIG. 20.

In block 2005, the method includes placing a reference electromagneticsensor 26 at or near the wellhead and one or more electromagneticsensors 26 and seimic sensors 28 at a distance from the wellhead. Thereference sensor may be an electromagnetic sensor 26. In other exampleembodiments, the reference sensor is a seismic sensor 28. In otherimplementations, electromagnetic or seismoelectric emissions are used asa time zero reference to which the signals from the one or moreelectromagnetic sensors 26 at a distance from the wellhead are comparedby the computer system 30.

In block 2010, the method further includes receiving a first set ofelectromagnetic signals generated by the electroseismic orseismoelectric conversion of seismic signals in the subterraneanformation that are caused, at least in part, by the productionoperation. These seismic signals may be referred to a passive sourceseismic signals.

In block 2015, the method includes cross correlating the signals fromthe reference sensor and the one or more electromagnetic sensors 26 andseismic sensors 28 located away from the wellhead. In certain exampleembodiment, the signal to noise ratio of monitoring the productionoperation is enhanced by correlating noisy targets with hydrocarbonproduction. The fluid pressure in the reservoir interval varies withproduction time and the number of producing wells. Although all theoverburden and basement are influenced by production pressure changes,the pressure changes in the reservoir create first-order electro-osmoticconversions. The mechanical noise from production operations travels atthe speed of sound through the rock. The fluid pressure noise travels atthe speed of pressure diffusion, which is much slower than the speed ofsound.

Based on the results of the cross correlation operation (block 2015),the computer system 30 then determines one or more reservoir propertiesin block 2020. I one example implementation, the computer system 30determines the connectedness of reservoir intervals. The pressurediffusion is indicative of the connectedness of the reservoir interval.In certain implementations, if two wells are not connected by acontinuous fluid path, then the pressure diffusion is cut off and nostreaming potential develops between wells. In certain implementations,a rapid drop of correlation amplitude with offset from the wellindicates a disconnected reservoir interval. In some situations, adisconnection might be caused by a fault or change in rock properties. Adisconnected interval stops enhanced oil recovery.

In certain implementations, the computer system 30 further accounts forthe frequency spectrum of the propagating pressure diffusion. The timelag between an event in a well and its mirror image in a distant part ofa reservoir is long. In addition, the frequency is shifted to lowerfrequencies by the high-frequency filtering of pressure diffusion. Atlarge distances from a well the information is lost. At shorterdistances, the frequency dependence is an indicator of permeability andproducibility.

Placement of Sensors

The embodiments of the present disclosure shown in FIGS. 3A, 3B, 13A,13B, 14A, and 14B, 16, 17, 18, 19A, 19B, and 20 may use an array ofelectromagnetic and seismic sensors. Other implementation may use onlyone or a small number of electromagnetic or seismic sensors. One exampleembodiment uses one or a small number of electromagnetic or seismicsensors to perform quality control for drilling a horizontal well. Insuch an implementation the intended path for the horizontal well ispresumably known. One electromagnetic or seismic sensor is placed at theintended well termination. As the drill progresses, the electromagneticsensor signal or the seismic sensor signal will increase in amplitudeand the arrival time will decrease. In some implementation, largechanges in amplitude may signify that the drill is passing through a wetzone, a change in lithology, or that it has moved outside of thereservoir. Non-monatomic progress in arrival time may signal deviationfrom the intended path. In other implementations, two or moreelectromagnetic or seismic sensors are placed along the prospectiveroute of the well. The data collected in this way will be complementaryto standard data collected during drilling.

In another implementation, one or more electromagnetic or seismicsensors are installed at fixed or several locations to indicate when anenhanced recovery operation has crossed a particular point in space. Forexample, in a water flood, it might be useful to know when the water isapproaching a producing well. In some implementations, anelectromagnetic or seismic sensor that is monitoring the reservoir willshow a rapid change in amplitude when the hydrocarbon/water interfacepasses beneath it.

Herein, “or” is inclusive and not exclusive, unless expressly indicatedotherwise or indicated otherwise by context. Therefore, herein, “A or B”means “A, B, or both,” unless expressly indicated otherwise or indicatedotherwise by context. Moreover, “and” is both joint and several, unlessexpressly indicated otherwise or indicated otherwise by context.Therefore, herein, “A and B” means “A and B, jointly or severally,”unless expressly indicated otherwise or indicated otherwise by context.

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the example embodiments herein that aperson having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend. Moreover, reference in the appended claims to an apparatusor system or a component of an apparatus or system being adapted to,arranged to, capable of, configured to, enabled to, operable to, oroperative to perform a particular function encompasses that apparatus,system, component, whether or not it or that particular function isactivated, turned on, or unlocked, as long as that apparatus, system, orcomponent is so adapted, arranged, capable, configured, enabled,operable, or operative.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the invention may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a tangible computer readable storage medium or any typeof media suitable for storing electronic instructions, and coupled to acomputer system bus. Furthermore, any computing systems referred to inthe specification may include a single processor or may be architecturesemploying multiple processor designs for increased computing capability.

Although the present invention has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims. Moreover, while thepresent disclosure has been described with respect to variousembodiments, it is fully expected that the teachings of the presentdisclosure may be combined in a single embodiment as appropriate.

What is claimed is:
 1. A method for monitoring a drilling operation in asubterranean formation, the method comprising: receiving, from a firstsensor array, one or more seismic signals caused, at least in part, bythe drilling operation in the subterranean formation; receiving, fromthe first sensor array, one or more electromagnetic signals generated byan electroseismic or seismoelectric conversion of the one or moreseismic signals caused, at least in part, by the drilling operation inthe subterranean formation; determining a property of one or more of thedrillstring and the subterranean formation based, at least in part, onthe seismic signals and the corresponding electromagnetic signalsreceived from the first sensor array; and wherein the first sensor arrayis arranged to monitor the drilling operation.
 2. The method of claim 1,wherein determining a property one or more of the drillstring and thesubterranean formation based, at least in part, on the seismic signalsand the corresponding electromagnetic signals received from the firstsensor array further comprises: determining a drill bit location based,at least in part, on the seismic signals and the correspondingelectromagnetic signals received from the first sensor array.
 3. Themethod of claim 1 wherein the first sensor array comprises a pluralityof seismic sensors and a plurality of electromagnetic sensors.
 4. Themethod of claim 1, wherein the first sensor array comprises a pluralityof geophones.
 5. The method of claim 1, wherein the drilling operationis to create a wellbore with at least one horizontal segment and atleast one vertical segment.
 6. The method of claim 1, wherein thedrilling operation is to create a wellbore with at least one verticalsegment.
 7. The method of claim 1, further comprising: receiving, from asecond sensor array, signals generated by the subterranean formation inresponse background noise or infrastructure; and removing at least somenoise from the signals from the first sensor array, based, at least inpart on the signals received from the second sensor array.
 8. The methodof claim 7, further comprising: receiving, from a third sensor array,one or more electromagnetic signals generated by the electroseismic orseismoelectric conversion of the seismic signals caused by the drillingoperation in the subterranean formation; wherein the third sensor arrayare located apart from the first sensor array; and wherein determining aproperty of one or more of the drillstring and the subterraneanformation based is further based, at least in part, on the receivedelectromagnetic signals from the third set of sensors.
 9. The method ofclaim 8, further comprising: performing a cross-correlation betweensignals from two sensors in one or more of the first sensor array, thesecond array of sensor, and the third sensor array.
 10. The method ofclaim 8, further comprising: performing an auto-correlation on at leastone signal from one sensors one of the first sensor array, the secondarray of sensor, and the third sensor array.
 11. The method of claim 1,wherein determining a property one or more of the drillstring and thesubterranean formation based, at least in part, on the signal receivedfrom the first sensor array, further comprises: applying one or morefilters to the signals received from the first sensor array to separateone or more surface waves from one or more seismic waves.
 12. The methodof claim 11, wherein applying one or more filters to the signalsreceived from the first sensor array to separate one or more surfacewaves from one or more seismic waves, further comprises: applying one ormore velocity filters to the signals received from the first sensorarray to separate one or more surface waves from one or more seismicwaves.
 13. The method of claim 11, wherein applying one or more filtersto the signals received from the first sensor array to separate one ormore surface waves from one or more seismic waves, further comprises:applying one or more spatial filters to the signals received from thefirst sensor array to separate one or more surface waves from one ormore seismic waves.
 14. The method of claim 1, further comprising:altering the drilling operation based, at least in part, on thedetermined a property of one or more of the drillstring and thesubterranean formation.
 15. The method of cause the drillstring toperform a controlled percussive drilling operation that is characterizedby a programmed percussive sequence; and, wherein determining a propertyof the formation based, at least in part, on the seismic signal and thecorresponding electromagnetic signals received from the first sensorarray comprises: performing a controlled percussive drilling operationthat is characterized by a programmed percussive sequence; anddetermining a property of the formation in front of the drill bit based,at least in part, on the seismic signals and the correspondingelectromagnetic signals received from the first sensor array.
 16. Themethod of claim 1, wherein determining a property of the formationbased, at least in part, on the seismic signal and the correspondingelectromagnetic signals received from the first sensor array comprises:performing a controlled percussive drilling operation that ischaracterized by a programmed percussive sequence; and determining aproperty of the formation below the drill bit based, at least in part,on the seismic signals and the corresponding electromagnetic signalsreceived from the first sensor array.
 17. The method of claim 1, whereindetermining a property of the formation based, at least in part, on theseismic signal and the corresponding electromagnetic signals receivedfrom the first sensor array comprises: performing a controlledpercussive drilling operation that is characterized by a programmedpercussive sequence; and imaging of the formation below the drill bitbased, at least in part, on the seismic signal and the correspondingelectromagnetic signals received from the first sensor array.
 18. Amethod for monitoring a drilling operation in a subterranean formation,the method comprising: receiving, at a time before the drillingoperation, first survey data from a first sensor array; processing thefirst survey data to determine one or more properties of the subsurfaceearth formation; and wherein the first survey data includes signalsgenerated by a subsurface earth formation in response to apassive-source electromagnetic signal, wherein the electromagneticsignal is generated by an electroseismic or seismoelectric conversion ofthe passive-source electromagnetic signal; wherein the passive-sourceelectromagnetic signal includes the earth's natural electromagneticfield and wherein the processing the first survey data to determine theone or more properties of the subsurface formation is based, at least inpart, on a correlation of the earth's electromagnetic field and theelectromagnetic signal generated by the electroseismic or seismoelectricconversion of the earth's natural electromagnetic field; and during thedrilling operation: receiving, from the first sensor array, one or moreseismic signals caused, at least in part, by the drilling operation inthe subterranean formation; receiving, from the first sensor array, oneor more electromagnetic signals generated by an electroseismic orseismoelectric conversion of the one or more seismic signals caused, atleast in part, by the drilling operation in the subterranean formation;and determining a property of one or more of the drillstring and thesubterranean formation based, at least in part, on the seismic signalsand the corresponding electromagnetic signals received from the firstsensor array.
 19. The method of claim 17, further comprising: receiving,at a time after the drilling operation, second survey data from thefirst sensor array; processing the first survey data and the secondsurvey data to determine one or more properties of the drillingoperation of subsurface earth formation; and wherein the first surveydata and second survey data include signals generated by a subsurfaceearth formation in response to a passive-source electromagnetic signal,wherein the electromagnetic signal is generated by an electroseismic orseismoelectric conversion of the passive-source electromagnetic signal;wherein the passive-source electromagnetic signal includes the earth'snatural electromagnetic field and wherein the processing the firstsurvey data and the second survey data to determine the one or moreproperties of the subsurface formation is based, at least in part, on acorrelation of the earth's electromagnetic field and the electromagneticsignal generated by the electroseismic or seismoelectric conversion ofthe earth's natural electromagnetic field.
 20. The method of claim 17,wherein: the one or more seismic signals during the drilling operationare caused, at least in part, by a controlled percussive drillingoperation that is characterized by a programmed percussive sequence, themethod further comprising: determining a property of the formationbased, at least in part, on the seismic signals and the correspondingelectromagnetic signals received from the first sensor array.
 21. Asystem comprising: a drillstring for performing a drilling operation ina subsurface formation, the drillstring including a drill bit; a firstsensor array to detect one or more seismic signals and one or moreelectromagnetic signals; a processor; a memory comprising non-transitoryexecutable instructions, that, when executed cause the processor to:receive, from the first sensor array, one or more seismic signalscaused, at least in part, by the drilling operation in the subterraneanformation; receive, from the first sensor array, one or moreelectromagnetic signals generated by an electroseismic or seismoelectricconversion of the one or more seismic signals caused, at least in part,by the drilling operation in the subterranean formation; determine aproperty of one or more of the drillstring and the subterraneanformation based, at least in part, on the seismic signals and thecorresponding electromagnetic signals received from the first sensorarray.
 22. The system of claim 21, wherein the executable instructionsfurther cause the processor to: determine a drill bit location based, atleast in part, on the seismic signals and the correspondingelectromagnetic signals received from the first sensor array.
 23. Thesystem of claim 21, wherein the first sensor array comprises a pluralityof geophones.
 24. The system of claim 21, further comprising: a secondsensor array to detect one or more seismic signals and one or moreelectromagnetic signals; and wherein the processor further operable to:receive, from the second sensor array, one or more seismic signalscaused, at least in part, by the drilling operation in the subterraneanformation; removing at least some noise from the signals from the firstsensor array, based, at least in part on the signals received from thesecond sensor array.
 25. The system of claim 21, further comprising: athird sensor array to detect one or more seismic signals and one or moreelectromagnetic signals, wherein the third sensor array are locatedapart from the first sensor array; and wherein the processor furtheroperable to: receive, from a third sensor array, one or moreelectromagnetic signals generated by the electroseismic orseismoelectric conversion of seismic signals caused by the drillingoperation in the subterranean formation; and wherein determining a drillbit location is further based, at least in part, on the receivedelectromagnetic signals from the third set of sensors.
 26. The system ofclaim 25, wherein the executable instructions further cause theprocessor to: perform a cross-correlation between signals from twosensors in one or more of the first sensor array, the second array ofsensor, and the third sensor array.
 27. The system of claim 25, whereinthe executable instructions further cause the processor to: perform anauto-correlation on at least one signal from one sensors one of thefirst sensor array, the second array of sensor, and the third sensorarray.
 28. The system of claim 25, wherein the executable instructionsfurther cause the processor to: alter the drilling operation based, atleast in part, on the determined a property of one or more of thedrillstring and the subterranean formation.
 29. The system of claim 21,wherein the executable instructions further cause the processor to:cause the drillstring to perform a controlled percussive drillingoperation that is characterized by a programmed percussive sequence; anddetermine a property of the formation in front of the drill bit based,at least in part, on the seismic signals and the correspondingelectromagnetic signals received from the first sensor array.
 30. Thesystem of claim 21, wherein the executable instructions further causethe processor to: cause the drillstring to perform a controlledpercussive drilling operation that is characterized by a programmedpercussive sequence; and determine a property of the formation below thedrill bit based, at least in part, on the seismic signals and thecorresponding electromagnetic signals received from the first sensorarray.